Multi-tank water heater systems

ABSTRACT

A water heater system including two or more water storage tanks plumbed in series, parallel or a combination thereof and methods of controlling said water heater systems. The water heater systems may include one or more mixing valves and one or more temperature sensors for providing water at a desired temperature.

This is a continuation application of co-pending U.S. patent applicationSer. No. 13/747,943, filed Jan. 23, 2013 and entitled MULTI-TANK WATERHEATER SYSTEMS, which is incorporated herein by reference.

TECHNICAL FIELD

The disclosure relates generally to water heaters, and moreparticularly, to water heater systems that have multiple tanks andmethods of controlling water heater systems having multiple tanks.

BACKGROUND

Water heaters are used in homes, businesses and in just about anyestablishment having the need for heated water. A conventional waterheater typically has at least one heating element or “heater,” such as agas-fired burner and/or electric heating element. Each water heater alsotypically has at least one thermostat or controller for controlling theheater. The controller typically receives signals related to thetemperature of the water within the water heater tank, often from atemperature sensor that is thermally engaged with the water in the waterheater tank.

In some instances, a water heater may operate in accordance with a firsttemperature set point and a second temperature set point. The differencebetween the first and second temperature set point may be referred to asthe temperature differential of the water heater. When temperaturesignals from the temperature sensor indicate that the water temperatureis below the first set point, for example when the water temperature isbelow about 120° F., the controller may turn on the heater and the waterwithin the water heater tank begins to heat. After some time, the watertemperature within the water heater tank will increase to the second setpoint, which, for example may be about 140° F. At this point, thecontroller may cause the heater to reduce its heat output or,alternatively, causes the heater to turn off. This heat cycle beginsagain when the water temperature within the water heater tank cools downbelow the first set point.

For a gas fired water heater, a temperature sensor, a gas valve and acontroller are often mounted relative to the water heater tank. Thecontroller typically receives a temperature signal from the temperaturesensor. The temperature sensor often protrudes into and is thermallycoupled to the water in the water heater tank. The controller typicallyis programmed to control the gas valve such that the temperature of thewater in the water heater tank remains between the first and secondtemperature set points, as described above. For an electric waterheater, a temperature sensor, a power delivery unit and a controller maybe mounted to the water heater tank. In this case, the controller maycontrol the power delivery unit such that the temperature of the waterin the water heater tank is kept between the first and secondtemperature set points.

Typically, a water heater is sized to service an expected peak hot waterdemand for a particular application. As such, for applications withhigher peak hot water demands, the size of the water heater may need tobecome fairly large. Larger water heaters, however, can be less energyefficient than smaller water heaters. What would be desirable,therefore, is a water heater system that can satisfy fairly large hotwater demands, but in a more energy efficient manner.

SUMMARY

The disclosure relates generally to water heaters, and moreparticularly, to water heater systems that have multiple tanks andmethods of controlling water heater systems having multiple tanks. Anillustrative but non-limiting example of the disclosure may be found ina water heater system that includes a first water storage tank having afirst water inlet and a first water outlet and a second water storagetank having a second water inlet and a second water outlet. A water linemay fluidly connect the first water outlet to the second water inlet fortransporting water from the first water storage tank to the second waterstorage tank. The system may further include a mixing valve having afirst inlet fluidly coupled to the second water outlet downstream of thesecond water storage tank and a cold water supply line in fluidcommunication with a second inlet of the mixing valve. A mixed wateroutlet may be fluidly coupled to an outlet of the mixing valve. Thesystem may further include a controller in electrical communication withthe mixing valve.

Another illustrative but non-limiting example of the disclosure may befound in a water heater system that includes a first water storage tankhaving a first water inlet and a first water outlet and a second waterstorage tank having a second water inlet and a second water outlet. Acold water supply line may be in fluid communication with the firstwater inlet and the second water inlet. A hot water line may be in fluidcommunication with the first water outlet and the second water outlet,wherein from the first water storage tank is mixed with water from thesecond water storage tank in the hot water line. The system may furtherinclude a mixing valve having a first inlet fluidly coupled to the hotwater supply line and a second cold water supply line in fluidcommunication with a second inlet of the mixing valve. A mixed wateroutlet may be fluidly coupled to an outlet of the mixing valve. Thesystem may further include a controller in electrical communication withthe mixing valve.

Another illustrative but non-limiting example of the disclosure may befound in a water heater system that includes a first water storage tankhaving a first water inlet and a first water outlet and a second waterstorage tank having a second water inlet and a second water outlet. Acold water supply line may be in fluid communication with the firstwater inlet of the first water storage tank and the second water inletof the second water storage tank. A first hot water supply line may bein fluid communication with the first water outlet of the first storagetank. A first mixing valve may be provided. The first mixing valve mayhave a first inlet fluidly coupled to the first hot water supply lineand a second inlet coupled to the cold water supply line. The firstmixing valve may be controllable via a first communications interface. Afirst mixed water outlet may be fluidly coupled to an outlet of thefirst mixing valve. A second hot water supply line may be in fluidcommunication with the second water outlet of the second storage tank. Asecond mixing valve may have a first inlet fluidly coupled to the secondhot water supply line and a second inlet coupled to the first mixedwater outlet of the first mixing valve. The second mixing valve beingcontrollable via a second communications interface. A second mixed wateroutlet may be fluidly coupled to an outlet of the second mixing valve,The system may further include a controller for controlling the firstmixing valve via the first communications interface and the secondmixing valve via the second communications interface.

Another illustrative but non-limiting example of the disclosure may befound in a water heater system that includes a first water storage tankhaving a first water inlet and a first water outlet, and a second waterstorage tank having a second water inlet and a second water outlet. Acold water supply line may be in fluid communication with the firstwater inlet of the first water storage tank and the second water inletof the second water storage tank. A first hot water supply line may bein fluid communication with the first water outlet of the first storagetank. A first mixing valve may have a first inlet fluidly coupled to thefirst hot water supply line and a second inlet coupled to the cold watersupply line. The first mixing valve may be controllable via a firstcommunications interface. A first mixed water outlet may be fluidlycoupled to an outlet of the first mixing valve. A second hot watersupply line may be in fluid communication with the second water outletof the second storage tank. A second mixing valve may have a first inletfluidly coupled to the second hot water supply line and a second inletcoupled to the cold water supply line. The second mixing valve may becontrollable via a second communications interface. A second mixed wateroutlet may be fluidly coupled to an outlet of the second mixing valve. Athird mixing valve may have a first inlet fluidly coupled to the firstmixed water output of the first mixing valve and a second inlet coupledto the second mixed water output of the second mixing valve. The thirdmixing valve may be controllable via a third communications interface. Athird mixed water outlet may be fluidly coupled to an outlet of thethird mixing valve. The system may further include a controller forcontrolling the first mixing valve via the first communicationsinterface, the second mixing valve via the second communicationsinterface and the third mixing valve via the third communicationsinterface.

Another illustrative but non-limiting example of the disclosure may befound in a water heater system that includes a cold water inlet line, ahot water outlet line, a first water heater having a first water inletand a first water outlet, and a second water heater having a secondwater inlet and a second water outlet. Configurable plumbing may becoupled to the cold water inlet line, the hot water outlet line, thefirst water inlet, the first water outlet, the second water inlet andthe second water outlet. The configurable plumbing may including one ormore controllable valves that, when in a first state, plumb the firstwater heater and the second water heater in a series configurationbetween the cold water inlet line and the hot water outlet line, andwhen in a second state, plumb the first water heater and the secondwater heater in a parallel configuration between the cold water inletline and the hot water outlet line. The system may further include acontroller for selectively switching the one or more controllable valvesbetween the first state and the second state.

Another illustrative but non-limiting example of the disclosure may befound in a method for controlling a water heater system having two ormore water storage tanks. The illustrative method may include: drawingwater from the water heater system; monitoring a first change intemperature of water in a first water storage tank; monitoring a secondchange in temperature of water in a second water storage tank;determining if the first change in temperature is different from thesecond change in temperature by at least a threshold amount; controllinga setpoint temperature of the first water storage tank and a setpointtemperature of the second water storage tank in accordance with a firstcontrol algorithm if the first change in temperature is different fromthe second change in temperature by at least the threshold amount; andcontrolling the setpoint temperature of the first water storage tank andthe setpoint temperature of the second water storage tank in accordancewith a second control algorithm that is different from the first controlalgorithm if the first change in temperature is not different from thesecond change in temperature by at least the threshold amount.

Another illustrative but non-limiting example of the disclosure may befound in a method for controlling a water heater system including two ormore water heater tanks. The method may include the steps ofestablishing a relationship between hot water usage and a time of day,and adjusting the temperature setpoint of a first water heater tankbased on the hot water usage for a given time period, and thetemperature setpoint of a second water heater tank may be adjusted basedon the hot water usage for the given time period.

BRIEF DESCRIPTION OF THE FIGURES

The following description should be read with reference to the drawings.The drawings, which are not necessarily to scale, depict examples andare not intended to limit the scope of the disclosure. The disclosuremay be more completely understood in consideration of the followingdescription of various examples in connection with the accompanyingdrawings, in which:

FIG. 1 is a schematic view of an illustrative but non-limiting waterheater system;

FIG. 2A is a schematic view of another illustrative but non-limitingwater heater system;

FIG. 2B is a schematic view of alternative to the illustrative butnon-limiting water heater system of FIG. 2A;

FIG. 3 is a schematic view of another illustrative but non-limitingwater heater system;

FIG. 4 is a schematic view of another illustrative but non-limitingwater heater system;

FIG. 5 is a schematic view of another illustrative but non-limitingwater heater system;

FIG. 6 is a schematic view of another illustrative but non-limitingwater heater system;

FIG. 7A is a schematic view of an illustrative but non-limiting waterheater system;

FIG. 7B is a schematic view of alternative to the illustrative butnon-limiting water heater system of FIG. 7A;

FIG. 8 is a schematic of an illustrative controller; and

FIG. 9 is a method of determining how multiple water tanks are plumbed.

While the disclosure is amenable to various modifications andalternative forms, specifics thereof have been shown by way of examplein the drawings and will be described in detail. It should beunderstood, however, that the intention is not to limit the disclosureto the particular examples described. On the contrary, the intention isto cover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the disclosure.

DESCRIPTION

The following description should be read with reference to the drawings,in which like elements in different drawings are numbered in likefashion. The drawings, which are not necessarily to scale, depictexamples that and are not intended to limit the scope of the disclosure.Although examples of construction, dimensions, and materials areillustrated for the various elements, those skilled in the art willrecognize that many of the examples provided have suitable alternativesthat may be utilized.

FIG. 1 is a schematic view of an illustrative but non-limiting waterheater system 10. Water heater system 10 includes a water tank 12. Thewater tank 12 may include an insulating layer (not explicitly shown)positioned about the water tank 12 to help reduce thermal losses fromthe water tank 12. Cold water enters water tank 12 through a cold waterline 14 and is heated by a gas burner 24. In some cases, the waterheater 10 may include an electric heating element rather than a gasburner 24. A power delivery unit (not shown) may be used to selectivelyapply power (i.e. current) to the electric heating element. Forgas-fired water heaters, a control unit 18 may control a gas controlunit, such as a gas valve, to regulate gas flow from a gas source 20through a combustion gas line 22 and into gas burner 24. A flue 26permits combustion byproducts to safely exit. In some instances, controlunit 18 may also control additional components of the water heatersystem. Whether oil, gas or electric powered, the resulting heated waterexits through a hot water line 16.

As can be seen, water heater 10 may include a temperature sensor 28 formonitoring the temperature of the water within the water tank 12. Insome cases, temperature sensor 28 may enter water tank 12 at a locationlaterally offset from control unit 18. In some instances, however,temperature sensor 28 may be located behind control unit 18, and in somecases, may be supported and retained by a common mounting bracket. Inany event, water tank 12 may include an aperture that is sized andconfigured to accept temperature sensor 28. The aperture may includethreads that are configured to accommodate corresponding matchingthreads on temperature sensor 28. In some cases, temperature sensor 28has a compression or frictional fit within the aperture. In otherinstances, water tank 12 may include a threaded spud (not explicitlyshown) that is configured to receive temperature sensor 28.

In some instances, the water tank 12 may be smaller than a typical waterheater tank (for example, in some instances, less than 60 gallons) for agiven application, while still providing the heater water capacity of alarger tank. In some instances, the setpoint temperature of the waterheater 10 may be set to a higher temperature than the temperaturedesired at the water outlet 40. The water heater 10 may include anelectronically controlled mixing valve 32 for injecting a regulatedamount of cold water into the hot water line 16 to achieve the desiredtemperature at the water outlet 40. The temperature of the hot waterexiting the water tank 12 may be hotter than the desired outlet water 40temperature, and thus the cold water may be used to temper, or cool, thehot water leaving the tank to achieve the desired outlet watertemperature. In some instances, the cold water line 14 may include abranch line 38 that may be directly connected to the mixing valve 32 forinjecting cold water into the mixing valve 32, while some cold waterpasses through cold water inlet 15 and into the water tank 12. In otherinstances, it is contemplated that a separate cold water line may beconnected to the mixing valve 32. In order to achieve mass flow balance,the flow rate of cold water in cold water line 14 may equal the flowrate of water exiting water outlet 40. The flow rate of cold waterentering the water tank 12 from the cold water inlet 15 may beapproximately equal to the flow rate of hot water exiting the water tank12 at the hot water line 16.

In some instances, water heater system 10 may be provided with a burner24 that is oversized for the water tank 12 relative to a conventionalwater heater. This may allow the water heater system 10 to more quicklyheat and maintain the water within the water tank 12 to a highertemperature than is desired at the water outlet 40, although this is notrequired. For example, if a water temperature of 110° F. is desired atthe water outlet 40, the water within the water tank 12 may bemaintained at a temperature of 135° F., although this is merelyexemplary. It is contemplated that the water temperature exiting thewater outlet 40 and the water temperature in the water tank 12 may beselected to be any value desired based on the desired application. Theoperating setpoint of the water temperature within the water tank 12 maybe selected based on a number of variables, such as, but not limited to,the temperature of the cold water entering the water tank 12 and themixing valve 32, the desired water temperature at the water outlet 40,the volume of the water tank 12, and/or the percent of usable hot waterin the water tank 12, etc. It is further contemplated that theproportion of cold water entering mixing valve 32 from the branch line38 to the hot water entering the mixing valve 32 from the hot water line16 may be dependent upon a number of variables, such as, but not limitedto, the temperature of the cold water entering the mixing valve 32, thetemperature of the hot water entering the mixing valve 32, and/or thedesired water temperature at the water outlet 40, etc.

The mixed water outlet 40 may include an outlet water temperature sensor34 for measuring the temperature of the water exiting the water heatersystem 10. In some embodiments, a temperature sensor 44 may be providedin the hot water line 16 and a temperature sensor 30 may be provided inthe cold water line 14 to help improve control of the water temperatureexiting the water heater system 10 at the outlet line 40. It iscontemplated that the mixing valve 32 and the outlet water temperaturesensor 34 may be connected to the control unit 18 via a communicationbus 42, or other alternative connection. For example, it is contemplatedthat the mixing valve 32 and temperature sensor 34 may be in wired orwireless communication with the control unit 18. Similarly, temperaturesensors 28, 30, 44 may also be in communication with the control unit 18via a communication bus 42 or other alternative connection, if present.It is contemplated that the temperature sensors 28, 30, 44 may be inwired or wireless communication with the control unit 18. The waterheater 10 may include an optional Man-Machine Interface (MMI) or userinterface 36. The user interface 36 may allow a user to view and/ormodify temperature setpoints and other control features, as desired.

Referring briefly to FIG. 8, it will be appreciated that control unit 18may include a controller. FIG. 8 is a block diagram of one suchcontroller 700. The controller 700 may be considered as being a portionof control unit 18, or separate from control unit 18. Controller 700 mayhave several blocks. In some cases, controller 700 may have anINPUT/OUTPUT block 710 that accepts signals from the temperaturesensors, such as temperature sensors 28, 30, 34, 44 (FIG. 1). If waterheater system 10 is in communication with an external thermostat orother controller, INPUT/OUTPUT block 710 may accommodateexternally-derived control signals, and/or provide status and/or otherinformation, as desired. In some cases, INPUT/OUTPUT block 710 mayprovide appropriate output command signals to an electrically controlledgas valve (not illustrated) within control unit 18, or otherelectrically controlled valve. For example, the INPUT/OUTPUT block 710may provide communication signals to the electronically controlledmixing valve 32 to change the ratio of hot water to cold water as neededto achieve the desired water temperature exiting the water outlet 40.

In some instances, controller 700 may include a microprocessor 720 thatmay be configured to accept appropriate signals from INPUT/OUTPUT block710, and to determine appropriate output signals that can be output viaINPUT/OUTPUT block 710, such as to other components within control unit18 (FIG. 1) and/or to an external thermostat or other controller.Microprocessor 720 may be programmed to accept a temperature signal fromone or more temperature sensors, such as temperature sensors 28, 30, 34,44 (FIG. 1), and to calculate or otherwise determine a commandtemperature to alter the temperature value received from the temperaturesensors 28, 30, 34, 44 in order to account or compensate for temperaturedifferentials and/or thermal lag caused by the partial thermal isolation(if present) of the temperature sensors 28, 30, 34, 44 from the water inthe water tank 12 and/or pipes 15, 16, 40. The microprocessor 720 may bealso be programmed to determine a command temperature based on thecurrent or expected hot water demand on the water heater system 10.While not explicitly illustrated, microprocessor 720 may also includememory and/or other components.

It is contemplated that the control unit 18 may include a controlalgorithm for operating the mixing valve 32 based on the desired watertemperature at the water outlet 40. For example, the control unit 18 mayinclude a controller configured to provide signals to the mixing valve32 via communication line 42 to change the ratio of hot water to coldwater based on feedback received from the user interface 36, the outletwater temperature sensor 34, the cold water temperature sensor 30 and/orthe hot water temperature sensor 44. While the mixing valve 32 has beendescribed as an electronically controllable mixing valve, it iscontemplated that the mixing valve 32 may be a non-electronicallycontrolled mixing valve (e.g. thermally controlled mixing valve).

The hot water capacity of the water tank 12 may depend on thetemperature setpoint of the water within the water tank 12, thetemperature of the cold water entering the water tank 12 through thecold water inlet 15, and the desired temperature of the water exitingthe water outlet 40. For example, given two water heater systems 10having the same temperature setpoint within the water tank 12, and thesame temperature of the cold water entering the water tank 12, thesystem having the lower water temperature exiting the water outlet 40will have a larger capacity. Likewise, given two water heater systems 10having different temperature setpoints within the water tank 12, and thesame temperature of the cold water entering the water tank 12 and thesame desired water temperature exiting the water outlet 40, the waterheater system with the higher temperature setpoint will have the largercapacity.

In some cases, a water heater system 10 having a water tank 12 may alsobe plumbed in series with an instantaneous water heater (not explicitlyshown). In some instances, the water tank 12 may be configured tomaintain the temperature of the water at a desired temperature. It iscontemplated that the temperature of the water in the water tank 12 maybe greater than, less than, or equal to the desired final watertemperature. When an instantaneous water heater is used in series with awater tank 12, the water may flow from the water outlet 40 and into theinstantaneous water heater. The instantaneous water heater may thenfurther heat the water, if necessary, to the desired water temperaturebefore delivering the water to the desired location. This may furtherincrease the effective hot water capacity of the water heater system 10.

It is contemplated that a variety of water tank configurations may beprovided to allow a water heater system to provide the hot watercapacity of a larger tank while being more energy efficient. FIG. 2A isa schematic view of another illustrative but non-limiting water heatersystem 100. The illustrative water heater system 100 of FIG. 2A mayinclude a first water tank 112 and a second water tank 130 plumbed inseries with one another. While the system 100 is illustrated as havingtwo tanks 112, 130, it is contemplated that the system 100 may includeany number of tanks desired, such as, but not limited to three, four, ormore. In some instances, the first water tank 112 may have a larger tankvolume than the second water tank 130. However, this is not required. Itis contemplated that the first water tank 112 and the second water tank130 may have the same volume or the second water tank 130 may have alarger tank volume than the first water tank 112. The first water tank112 and second water tank 130 may be standard (e.g. non-condensing)atmospheric water tanks or any form of storage water tanks as desired.In some instances, the first water tank 112 and the second water tank130 may be physically separate structures as shown in FIG. 2A. In otherinstances, the first water tank 112 and the second water tank 130 may befluidly isolated volumes that are part of the same assembly. In someembodiments, the first water tank 112 and/or second water tank 130 maybe replaced with a tankless (or instantaneous) water heater.

The first water tank 112 may include an insulating layer (not explicitlyshown) positioned about the water tank 112 to help reduce thermal lossesfrom the water tank 112. Cold water enters water tank 112 through a coldwater line 114 and is heated by a gas burner 124. In some cases, thewater tank 112 may include an electric heating element rather than a gasburner 124. When so provided, a power delivery unit (not shown) may beused to selectively apply power (i.e. current) to the electric heatingelement. For gas-fired water heaters, a control unit 118 may control agas control unit, such as a gas valve, to regulate gas flow from a gassource 120 through a combustion gas line 122 and into gas burner 124. Insome instances, control unit 118 may also control additional componentsof the water heater system. A flue 126 permits combustion byproducts tosafely exit. The resulting heated water exits through a water outletline 116.

Similarly, the second water tank 130 may include an insulating layer(not explicitly shown) positioned about the water tank 130 to helpreduce thermal losses from the water tank 130. Warm and/or hot water mayenter the second water tank 130 from the first water tank 112 throughthe water outlet line 116 and is heated by a gas burner 140. In somecases, the water tank 130 may include an electric heating element ratherthan a gas burner 140. When so provided, a power delivery unit (notshown) may be used to selectively apply power (i.e. current) to theelectric heating element. For gas-fired water heaters, a control unit134 may control a gas control unit, such as a gas valve, to regulate gasflow from a gas source 136 through a combustion gas line 138 and intogas burner 140. In some instances, control unit 134 may also controladditional components of the water heater system. A flue 142 permitscombustion byproducts to safely exit. In either case, the resultingheated water exits through a hot water line 132.

In some cases, the first water tank 112 may include a temperature sensor128. In some cases, temperature sensor 128 may enter water tank 112 at alocation laterally offset from control unit 118. In some instances,however, temperature sensor 128 may instead be located behind controlunit 118 and in some cases, may be supported and retained by a commonmounting bracket. In any event, water tank 112 may include an aperturethat is sized and configured to accept temperature sensor 128. Theaperture may include threads that are configured to accommodatecorresponding matching threads on temperature sensor 128. In some cases,temperature sensor 128 has a compression or frictional fit within theaperture. In other instances, water tank 112 may include a threaded spud(not explicitly shown) that is configured to receive temperature sensor128.

Similarly, the second water tank 130 may include a temperature sensor144. In some cases, temperature sensor 144 may enter water tank 130 at alocation laterally offset from control unit 134. In some instances,however, temperature sensor 144 may instead be located behind controlunit 134 and in some cases, may be supported and retained by a commonmounting bracket. In any event, water tank 130 may include an aperturethat is sized and configured to accept temperature sensor 144. Theaperture may include threads that are configured to accommodatecorresponding matching threads on temperature sensor 144. In some cases,temperature sensor 144 has a compression or frictional fit within theaperture. In other instances, water tank 130 may include a threaded spud(not explicitly shown) that is configured to receive temperature sensor144.

The first water tank 112 may be hooked up to a cold water line 114 forreceiving cold water at a first temperature from a water source. The gasburner 124 may heat the cold water entering the first water tank 112 toa first predetermined temperature setpoint. It is contemplated that thetemperature setpoint of the first water tank 112 may be greater than thetemperature of the cold water entering the first water tank 112 but lessthan the desired output temperature of the water heater system 100. Insome instances, the temperature setpoint of the first tank 112 may beequal to or greater than the desired output temperature of the waterheater system 100. It is further contemplated that during periods of lowhot water usage, the water in the first tank 112 may be allowed to warmto room temperature without the use of the burner 124. The heated watermay exit the first water tank 112 through the water outlet line 116 andflow into the second water tank 130. The gas burner 140 may heat thewater entering the second water tank 130 to a second predeterminedtemperature setpoint. It is contemplated that the second predeterminedtemperature setpoint may be greater than the first predeterminedtemperature setpoint of the first water tank 112. In some instances, thesecond predetermined temperature setpoint may be the desired hot watersetpoint for the water heater system 100. It is contemplated that bymaintaining the water temperature of the first water tank 112 at a lowertemperature than the hot water setpoint for the water heater system 100,water heater system 100 may reduce standby losses and the overall waterheater system 100 efficiency factor (EF) rating may be improved.

In some instances, the gas burner 124 on the first water tank 112 may bea standard size for the size of the water tank 112 and the gas burner140 on the second water tank 130 may be oversized (not explicitly shown)for the size of the water tank 130. This may allow the gas burner 140 inthe second water tank 130 to more quickly elevate the water temperatureof the incoming water from the water outlet line 116 of the first tank112 to the setpoint water temperature of the water heater system 100,without sacrificing the overall hot water capacity of the system.However, this is not required. It is contemplated that the reverseconfiguration may be used or both the first water tank 112 and thesecond water tank 130 may have standard size burners 124, 140 oroversized burners for the size of water tanks 112, 130, as desired.

In some instances, the gas burner 124 on the first water tank 112 may becontrolled by the control unit 118 and the gas burner 140 on the secondwater tank 130 may be controlled by the control unit 134. Thetemperature sensors 128, 144 may be in communication with control units118, 134 via a communication bus 146 or other alternative connection,wired or wireless. The control unit 118 and the control unit 134 may bein either wired or wireless communication with one another to facilitatecoordinated control of the entire water heater system 100. In such anarrangement, for example, one control may act as a master and the otheras a slave. This communication may allow for sharing of watertemperature, safety status, water temperature stacking, etc. If amaster/slave protocol is used, the master may be allowed to increase ordecrease the temperature setpoint(s) of itself and the slave controls.In some instances, the master control may also be allowed to lockout anyof the controls, depending on the safety status that was communicated tothe master from the slave(s). It is also contemplated that a singleoverall control module (not explicitly shown) may be used for both tanks112, 130, especially if both tanks 112, 130 are part of the sameassembly (not explicitly shown). It is further contemplated that, whilenot explicitly shown, the water heater system 100 may include a userinterface to allow a user to view and/or change the water heater system100 parameters, such as, but not limited to, temperature setpoints ofthe first water tank 112 and the second water tank 130, schedule ofexpected high hot water demand periods, etc.

Referring briefly to FIG. 8, it will be appreciated that control units118, 134 may include controllers. FIG. 8 is a block diagram of such acontroller 700. The controller 700 may be considered as being a portionof control units 118, 134, or separate from control units 118, 134. Itis contemplated that the water heater system 100 may be controlled by analgorithm in such a way as to increase the efficiency while maintainingor increasing hot water capacity. For example, when the controller 700senses or detects a water draw in the second water tank 130, thecontroller may activate both the gas burner 124 and the gas burner 140,sometimes simultaneously, to help increase the hot water capacity of thewater heater system 100, even though the temperature sensor 128 in thefirst water tank 112 may not sense the water draw via its temperaturesensor for several minutes due to its larger size. This rapid turn-onalgorithm may help reduce the drop in water temperature in the firstwater tank 112 that would otherwise occur with a control that is notconfigured to rapid-fire the first water tank 112. Optionally, thecontrol algorithm may increase the setpoint temperature of the firstwater tank 112, at least temporarily after the second water tank 130senses a draw on the water supply to provide more capacity.

In some instances, the first water tank 112 may include a flue gascondensing heat exchanger, as illustrated in FIG. 2B. That is, the fluegas exiting the second water tank 130 may be directed through the flue142 and into a flue gas condensing heat exchanger (not explicitly shown)in the first water tank 112. It is contemplated that the heat from theflue gas in flue 142 may be used in addition to, or in place of, the gasburner 124 of the first water tank 112. If the flue gas heat exchangeris used in place of the gas burner 124, it is contemplated that the gasburner 124, combustion gas line 122, gas source 120, and flue 126 in thefirst water tank 112 may not be provided.

FIG. 3 illustrates another exemplary water heater system 200. The waterheater system 200 may include a first water tank 212 and a second watertank 230 plumbed in series with one another as shown. While the waterheater system 200 is illustrated as having two tanks 212, 230, it iscontemplated that the water heater system 200 may include any number oftanks desired, such as, but not limited to three, four, or more. In someembodiments, the first and second water tanks 212, 230 may be twosmaller tanks (for example, the tanks 212, 230 may each be less than 60gallons, less than 50 gallons, less than 40 gallons, less than 30gallons, etc.). It is contemplated that the first water tank 212 and thesecond water tank 230 may have the same volume. However, this is notrequired. It is contemplated that in some instances the first water tank212 may have a larger tank volume than the second water tank 230, or thesecond water tank 230 may have a larger tank volume than the first watertank 212. The first water tank 212 and second water tank 230 may bestandard (e.g. non-condensing) atmospheric water tanks or any form ofstorage water tanks desired. In some instances, the first water tank 212and the second water tank 230 may be physically separate structures asshown in FIG. 3. In other instances, the first water tank 212 and thesecond water tank 230 may be fluidly isolated volumes that are part ofthe same assembly. In some embodiments, the first water tank 212 and/orsecond water tank 230 may be replaced with a tankless (or instantaneous)water heater.

The first water tank 212 may include an insulating layer (not explicitlyshown) positioned about the water tank 212 to help reduce thermal lossesfrom the water tank 212. Cold water enters water tank 212 through a coldwater line 214 and is heated by a gas burner 224. In some cases, thewater tank 212 may include an electric heating element rather than a gasburner 224. When so provided, a power delivery unit (not shown) may beused to selectively apply power (i.e. current) to the electric heatingelement. For gas-fired water heaters, a control unit 218 may control agas control unit, such as a gas valve, to regulate gas flow from a gassource 220 through a combustion gas line 222 and into gas burner 224. Insome instances, control unit 218 may also control additional componentsof the water heater system. A flue 226 permits combustion byproducts tosafely exit. Regardless of the power source of the first water tank 212(gas, oil, electric, etc.), the resulting heated water exits through awater outlet line 216.

Similarly, the second water tank 230 may include an insulating layer(not explicitly shown) positioned about the water tank 230 to helpreduce thermal losses from the water tank 230. Warm and/or hot water mayenter the second water tank 230 from the first water tank 212 throughthe water outlet line 216 and is heated by a gas burner 240. In somecases, the water tank 230 may include an electric heating element ratherthan a gas burner 240. When so provided, a power delivery unit (notshown) may be used to selectively apply power (i.e. current) to theelectric heating element. For gas-fired water heaters, a control unit234 may control a gas control unit, such as a gas valve, to regulate gasflow from a gas source 236 through a combustion gas line 238 and intogas burner 244. In some instances, control unit 234 may also controladditional components of the water heater system. A flue 242 permitscombustion byproducts to safely exit. Regardless of the power source ofthe first water tank 230 (gas, oil, electric, etc.), the resultingheated water exits through a hot water line 232.

The first water tank 212 may include a temperature sensor 228. In somecases, temperature sensor 228 may enter water tank 212 at a locationlaterally offset from control unit 218. In some instances, however,temperature sensor 228 may instead be located behind control unit 218and in some cases, may be supported and retained by a common mountingbracket. In any event, water tank 212 may include an aperture that issized and configured to accept temperature sensor 228. The aperture mayinclude threads that are configured to accommodate correspondingmatching threads on temperature sensor 228. In some cases, temperaturesensor 228 has a compression or frictional fit within the aperture. Inother instances, water tank 212 may include a threaded spud (notexplicitly shown) that is configured to receive temperature sensor 228.

Similarly, the second water tank 230 may include a temperature sensor244. In some cases, temperature sensor 244 may enter water tank 230 at alocation laterally offset from control unit 234. In some instances,however, temperature sensor 244 may instead be located behind controlunit 234 and in some cases, may be supported and retained by a commonmounting bracket. In any event, water tank 230 may include an aperturethat is sized and configured to accept temperature sensor 244. Theaperture may include threads that are configured to accommodatecorresponding matching threads on temperature sensor 244. In some cases,temperature sensor 244 has a compression or frictional fit within theaperture. In other instances, water tank 230 may include a threaded spud(not explicitly shown) that is configured to receive temperature sensor244.

The first water tank 212 may be hooked up to a cold water line 214 forreceiving cold water from a water source. In some instances, the coldwater line 214 may include a branch line 246 directly connected to anelectronically controlled mixing valve 254 for injecting some cold waterinto the mixing valve 254, as will be discussed in more detail below,while some cold water passes through cold water inlet 248 and into thefirst water tank 212. The gas burner 224 may heat the cold waterentering the first water tank 212 to a first predetermined temperaturesetpoint. It is contemplated that the temperature setpoint of firstwater tank 212 may be greater than the temperature of the cold waterentering the first water tank 212 but less than the desired outputtemperature of the water heater system 200 at the mixed water outletline 258. In some instances, the temperature setpoint of the first tank212 may be equal to or greater than the desired output temperature ofthe water heater system 200. It is contemplated that during periods oflow water usage, the water within the first water tank 212 may beallowed to warm to room temperature without the use of the burner 224 tohelp conserve energy.

The heated water may exit the first water tank 212 through the wateroutlet line 216 and flow into the water inlet of the second water tank230. The gas burner 240 of the second water tank 230 may heat the waterentering the second water tank 230 to a second predetermined temperaturesetpoint. It is contemplated that the second predetermined temperaturesetpoint may be greater than the first predetermined temperaturesetpoint of the first water tank 212, although this is not required. Insome instances, the second predetermined temperature setpoint may be thedesired hot water setpoint for the water heater system 200. In otherinstances, the second predetermined temperature setpoint may be higherthan the desired water temperature exiting the water heater system 200at the water outlet line 258. It is contemplated that by maintaining thewater temperature of the first water tank 212 at a lower temperature,the water heater system 200 may reduce standby losses and the overallwater heater system 200 efficiency rating may be improved.

Heated water may exit the second water tank 230 through hot water line232. The water heater system 200 may include an electronicallycontrolled mixing valve 254 for injecting a regulated amount of coldwater into the hot water line 232 to achieve a desired temperature atthe water outlet 258. As noted above, the temperature of the hot waterexiting the second water tank 230 may be hotter than the desired outlet258 water temperature and thus the cold water may be used to temper, orcool, the hot water leaving the tank to achieve the desired outlettemperature. While the system as shown as having a branch cold waterline 246, a separate cold water line may be connected directly to themixing valve 254.

It is contemplated that the temperature of the water exiting the wateroutlet 258 and the temperature setpoint of the first and second watertanks 212, 230 may be selected to be any value desired based on theparticular application. The operating setpoint of the water tanks 212,230 may be selected based on a number of variables, such as, but notlimited to, the temperature of the cold water entering the first watertank 212, the desired water temperature at the water outlet 258, thevolume of the water tanks 212, 230 and/or the percent of usable hotwater in the water tanks 212, 230, etc. It is further contemplated thatthe proportion of cold water entering mixing valve 254 from the branchline 246 to the hot water entering the mixing valve 254 from the hotwater line 232 may be dependent upon a number of variables, such as, butnot limited to, the temperature of the cold water entering the mixingvalve 254, the temperature of the hot water entering the mixing valve254, and/or the desired water temperature at the water outlet 258, etc.In some cases, the operating setpoints and/or the proportion of coldwater entering mixing valve 254 may be dynamic, and may vary with time,the temperature of the cold water entering the first water tank 212, thedesired water temperature at the water outlet 258, the volume of thewater tanks 212, 230, the percent of usable hot water in the water tanks212, 230, and/or any other variable, as desired.

In some instances, the gas burner 224 on the first water tank 212 may beof a standard size for the size of the water tank 212 (e.g. standardBTU/gallon), and the gas burner 240 on the second water tank 230 may beoversized for the size of the water tank 230 (larger BTU/gallon). Thatis, and in some cases, the gas burner 224 may have a first heat outputand the gas burner 240 may have a second heat output, wherein the ratioof the second heat output to the water volume of the second water tank230 may be greater than the ratio of the first heat output to the watervolume of the first water tank 212. This may help the gas burner 240 inthe second water tank 230 to more quickly elevate the water temperatureof the incoming water from the water outlet line 216 to the setpointwater temperature without sacrificing the overall hot water capacity ofthe system. However, this is not required. It is contemplated that thereverse configuration may be used or both the first water tank 212 andthe second water tank 230 may have standard size or oversized burners224, 240 for the size of the water tanks 212, 230.

The mixed water outlet 258 may include an outlet water temperaturesensor 256 for measuring the temperature of the water exiting the waterheater system 200. In some embodiments, a temperature sensor 252 may beprovided in the hot water line 232 and a temperature sensor 250 may beprovided in the cold water line 214 to help improve control of the watertemperature exiting the water heater system 200 at the outlet line 258.It is contemplated that the mixing valve 254 and the outlet watertemperature sensor 256 may be connected to the control units 218, 234via a communication bus 262, or other alternative connection. Forexample, it is contemplated that the mixing valve 254 and temperaturesensor 256 may be in wired or wireless communication with the controlunits 218, 234. Similarly, temperature sensors 228, 244, 250, 252 may bein communication with the control units 218, 234 via a communication bus262 or other alternative connection. It is contemplated that thetemperature sensors 228, 244, 250, 252 may be in wired or wirelesscommunication with the control units 218, 234. The water heater system200 may include an optional Man-Machine Interface (MMI) or userinterface 260. The user interface 260 may allow a user to view and/ormodify temperature setpoints and other control features.

In some instances, the gas burner 224 on the first water tank 212 may becontrolled by control unit 218 and the gas burner 240 on the secondwater tank 230 may be controlled by control unit 234. The control unit218 and the control unit 234 may be in either wired or wirelesscommunication with one another to facilitate control of the entire waterheater system 200. In such an arrangement, one control may act as masterand the other as slave. This communication may allow for sharing ofwater temperature, safety status, water temperature stacking, etc. If amaster/slave protocol is used, the master may be allowed to increase ordecrease the temperature setpoint(s) of itself and the slave controls.In some instances, the master control may also be allowed to lockout anyof the controls (master and/or slave controls), depending on the safetystatus that was communicated to the master. It is also contemplated thata single overall control module (not explicitly shown) may be used forboth tanks 212, 230, especially if both tanks 212, 230 are part of thesame assembly (not explicitly shown).

Referring briefly to FIG. 8, it will be appreciated that control units218, 234 may include controllers. FIG. 8 is a block diagram of such acontroller 700. The controller 700 may be considered as being a portionof control units 218, 234, or separate from control units 218, 234.Controller 700 may have several blocks. In some cases, controller 700may have an INPUT/OUTPUT block 710 that accepts signals from thetemperature sensors 228, 244, 250, 252, 256 (FIG. 3). If water heatersystem 200 is in communication with an external thermostat or othercontroller, INPUT/OUTPUT block 710 may accommodate externally-derivedcontrol signals, and/or provide status and/or other information, asdesired. In some cases, INPUT/OUTPUT block 710 may also provideappropriate output command signals to an electrically controlled gasvalve (not illustrated) within control units 218, 234, or otherelectrically controlled valve, such as mixing valve 254.

In some instances, controller may include a microprocessor 720 that maybe configured to accept appropriate signals from INPUT/OUTPUT block 710,and to determine appropriate output signals that can be outputted viaINPUT/OUTPUT block 710, such as to other components within control units218, 234 (FIG. 3) and/or to an external thermostat or other controller.Microprocessor 720 may be programmed to accept a temperature signal fromtemperature sensors 228, 244, 250, 252, 256 (FIG. 3), and to calculateor otherwise determine a command temperature that alters the temperaturevalue received from the temperature sensors 228, 244, 250, 252, 256 inorder to account or compensate for temperature differentials and/orthermal lag caused by the partial thermal isolation (if present) of thetemperature sensors 228, 244, 250, 252, 256 from the water in the watertanks 212, 230 or water lines 214, 232, 258. The microprocessor 720 maybe also be programmed to determine a command temperature based on thecurrent or expected hot water demand on the water heater system 200.While not explicitly illustrated, microprocessor 720 may also includememory and/or other components.

It is contemplated that one or both of the control units 218, 234 mayinclude a control algorithm for operating the mixing valve 254 based onthe desired water temperature at the water outlet 258. For example, thecontrol units 218, 234 may include a controller configured to providesignals to the mixing valve 254 via communication line 262 to change theratio of hot water to cold water based on feedback received from theuser interface 260, the outlet water temperature sensor 256, the coldwater temperature sensor 250, and/or the hot water temperature sensor252. While the mixing valve 254 has been described as an electronicallycontrollable mixing valve, it is contemplated that the mixing valve 254may be a non-electronically controlled mixing valve (e.g. a thermallycontrolled mixing valve). It is contemplated that one or more mixingvalves 254 may be used with any size of tank and with any number oftanks.

The hot water capacity may depend on the temperature setpoint of thewater within the water tanks 212, 230, the temperature of the cold waterentering the first water tank 212 through the cold water inlet 248, thetemperature of the water exiting the first water tank 212 through thewater line 216, the temperature of the water exiting the second watertank 230 through the water line 232 and the desired temperature of thewater exiting the water outlet 258.

In some instances, the water heater system 200 may be controlled by analgorithm in such a way as to increase the efficiency while maintainingor increasing hot water capacity. For example, the instant that thesecond water tank 230 senses a draw on the water supply, both the firstgas burner 224 and the second gas burner 240 may fire simultaneously tohelp maintain the hot water capacity of the system, even though thetemperature sensor 228 in the first water tank 212 may not sense thedraw for several minutes. This rapid turn-on algorithm may help reducethe drop in water temperature in the first water tank 212 that wouldotherwise occur with a control that is not configured to rapid-fire thefirst water tank 212. Optionally, the control algorithm may increase thesetpoint temperature of the first water tank 212, at least temporarilyafter the second water tank 230 senses a draw on the water supply toprovide more capacity.

While not explicitly shown, it is further contemplated that in someinstances, the first water tank 212 may include a flue gas condensingheat exchanger. In some embodiments, the flue gas exiting the secondwater tank 230 may be directed through the flue 242 and into a heatexchanger (not explicitly shown) in the first water tank 212. It iscontemplated that the heat from the flue gas in flue 242 may be used inaddition to, or in place of, the gas burner 224. If the flue gas heatexchanger is used in place of the gas burner 224, it is contemplatedthat the gas burner 224, combustion gas line 222, gas source 220, andflue 226 in the first water tank 212 may not be necessary. The reverseconfiguration may also be used.

Referring to FIGS. 2A, 2B, and 3, in some instances, the water heatersystem 100, 200 may use the relationship between hot water usage and thetime of day to determine the most efficient way to heat the first andsecond tanks 112, 212, 130, 230. In some embodiments, the relationshipbetween hot water usage and the time of day may be entered into the userinterface 260, if so provided, directly by a user. In other embodiments,the controller(s) 118, 134, 218, 234 may utilize an algorithm thatmonitors hot water usage to develop a schedule of expected hot waterdemand. For systems without a water flow rate sensor, the controller(s)118, 134, 218, 234 may indirectly estimate hot water usage by monitoringthe water temperature and/or call-for-heat signals. For systems with awater flow rate sensor, the controller(s) 118, 134, 218, 234 may usethat signal to directly measure hot water usage over time. Thisinformation may be used to construct a schedule of expected hot waterdemand over time.

For water heaters with an integrated controller, the controllers of eachwater heater may communicate as master/slaves, as discussed above, orvia another communication protocol, to work together to adjust thetemperature setpoint of each water tank 112, 130, 212, 230. For waterheater systems without an integrated controller, a single controller maybe used to control the temperature set points of all of the water tanks.Based on the configuration of the water heaters (e.g. series orparallel), the required hot-water capacity at a given time of day, andwhether there are any mixing or shutoff valves at the water heateroutlets, the temperature setpoints of each of the water tanks 112, 130,212, 230 may be optimized such that the water tanks 112, 130, 212, 230as a group meet the immediate hot-water demands of the user.

In one example, a system including two tanks in series, such as tanks112, 130 or tanks 212, 230, the first tank 112, 212 may receive coldwater with a first temperature T1 and may heat the water to a secondtemperature T2. For illustrative purposes only, the first and secondtanks 112, 212, 130, 230 may each have a volume of 30 gallons. Thesecond tank 130, 230 receives heated water from the first tank 112, 212and heats the water to the desired temperature T3, which may be greaterthan T2. The water from the second tank 130, 230 may be output to thehot water distribution system. If the predicted hot water demand for theuser at the current time is 10 gallons, the master control may reducethe temperature setpoint of the first tank 112, 212 to T1 and leave thetemperature setpoint of the second tank 130, 230 at a temperature thatwould provide 10 gallons of water at temperature T3. Such a controlalgorithm may prevent the burner 124, 224 on the first tank 112, 212from heating the water in the first tank 112, 212 unnecessarily. If,however, the predicted hot water demand for the user at the current timeis 45 gallons, the master control may change the temperature setpoint ofthe first tank 112, 212 to just below T3 and leave the temperaturesetpoint of the second tank 130, 230 at T3 or above. This may allow thesystem 100, 200 to provide more than 30 gallons of water without heatingboth the first tank 112, 212 and the second tank 130, 230 to the desiredtemperature T3 at all times.

FIG. 4 illustrates another exemplary water heater system 300. The waterheater system 300 may include a first water tank 312 and a second watertank 330 plumbed in parallel with one another. While the system 300 isillustrated as having two tanks 312, 330, it is contemplated that thesystem 300 may include any number of tanks desired, such as, but notlimited to three, four, or more. In some embodiments, the first andsecond water tanks 312, 330 may be two smaller tanks (for example, thetanks 312, 330 may each be less than 60 gallons, less than 50 gallons,less than 40 gallons, less than 30 gallons, etc.). It is contemplatedthat the first water tank 312 and the second water tank 330 may have thesame volume. However, this is not required. In some instances the firstwater tank 312 may have a larger tank volume than the second water tank330, or the second water tank 330 may have a larger tank volume than thefirst water tank 312. The first water tank 312 and the second water tank330 may be standard (e.g. non-condensing) atmospheric water tanks, orany form of storage water tanks desired. In some instances, the firstwater tank 312 and the second water tank 330 may be physically separatestructures as shown in FIG. 4. In other instances, the first water tank312 and the second water tank 330 may be fluidly isolated volumes thatare part of the same assembly. In some embodiments, the first water tank312 and/or second water tank 330 may be replaced with a tankless (orinstantaneous) water heater.

The first water tank 312 may include an insulating layer (not explicitlyshown) positioned about the water tank 312 to help reduce thermal lossesfrom the water tank 312. Cold water enters water tank 312 through a coldwater line 314 and is heated by a gas burner 324. In some cases, thewater tank 312 may include an electric heating element rather than a gasburner 324. When so provided, a power delivery unit (not shown) may beused to selectively apply power (i.e. current) to the electric heatingelement. For gas-fired water heaters, a control unit 318 may control agas control unit, such as a gas valve, to regulate gas flow from a gassource 320 through a combustion gas line 322 and into gas burner 324. Insome instances, control unit 318 may also control additional componentsof the water heater system. A flue 326 permits combustion byproducts tosafely exit. Regardless of the power source of the first water tank 312(gas, oil, electric, etc.), the resulting heated water exits through awater outlet line 316.

Similarly, the second water tank 330 may include an insulating layer(not explicitly shown) positioned about the water tank 330 to helpreduce thermal losses from the water tank 312. Cold water may enter thesecond water tank 330 through a cold water line 350 branched off of acold water line 314 and is heated by a gas burner 340. In someinstances, cold water may be supplied to the second water tank 330through a separate cold water supply. In some cases, the water tank 330may include an electric heating element rather than a gas burner 340.When so provided, a power delivery unit (not shown) may be used toselectively apply power (i.e. current) to the electric heating element.For gas-fired water heaters, a control unit 334 may control a gascontrol unit, such as a gas valve, to regulate gas flow from a gassource 336 through a combustion gas line 338 and into gas burner 344. Insome instances, control unit 334 may also control additional componentsof the water heater system. A flue 342 permits combustion byproducts tosafely exit. Regardless of the power source of the first water tank 330(gas, oil, electric, etc.), the resulting heated water exits through awater outlet line 332.

The first water tank 312 may include a temperature sensor 328. In somecases, temperature sensor 328 may enter water tank 312 at a locationlaterally offset from control unit 318. In some instances, however,temperature sensor 328 may instead be located behind control unit 318and in some cases, may be supported and retained by a common mountingbracket. In any event, water tank 312 may include an aperture that issized and configured to accept temperature sensor 328. The aperture mayinclude threads that are configured to accommodate correspondingmatching threads on temperature sensor 328. In some cases, temperaturesensor 328 has a compression or frictional fit within the aperture. Inother instances, water tank 312 may include a threaded spud (notexplicitly shown) that is configured to receive temperature sensor 328.

Similarly, the second water tank 330 may include a temperature sensor344. In some cases, temperature sensor 344 may enter water tank 330 at alocation laterally offset from control unit 334. In some instances,however, temperature sensor 344 may instead be located behind controlunit 334 and in some cases, may be supported and retained by a commonmounting bracket. In any event, water tank 330 may include an aperturethat is sized and configured to accept temperature sensor 344. Theaperture may include threads that are configured to accommodatecorresponding matching threads on temperature sensor 344. In some cases,temperature sensor 344 has a compression or frictional fit within theaperture. In other instances, water tank 330 may include a threaded spud(not explicitly shown) that is configured to receive temperature sensor344.

Both the first water tank 312 and the second water tank 330 may behooked up to a cold water line 314 for receiving cold water from a watersource. In some instances, the cold water line 314 may include a branchline 346 directly connected to an electronically controlled mixing valve356 for injecting some cold water into the mixing valve 356, as will bediscussed in more detail below, while some cold water passes throughanother portion of the line 348 and enters the tank through the coldwater inlet 366. In some embodiments, the cold water line 314 may alsoinclude a second cold water branch line 350 for delivering cold water tothe second water tank 330. In other embodiments, a separate cold watersupply line may be used to deliver water to the second water tank 330.The first gas burner 324 may heat the cold water entering the firstwater tank 312 to a first predetermined temperature setpoint, and thesecond gas burner 340 may heat the cold water entering the second watertank 330 to a second predetermined temperature setpoint. In someembodiments, the first and second temperature setpoints may be equal orapproximately equal; however, this is not required. It is contemplatedthat the temperature setpoint of the first and second water tanks 312,330 may be equal to or greater than the desired water temperature at thewater outlet 360. In some cases, the first and second temperaturesetpoints may be different.

Heated water may exit the first water tank 312 through hot water line316. Heated water may also exit the second water tank 330 through hotwater line 332. The hot water lines 316, 332 may join together at anintersection 368. The hot water from lines 316, 332 may mix and bedelivered to an electronically controlled mixing valve 356 via hot waterline 370. The electronically controlled mixing valve 356 may inject aregulated amount of cold water into the hot water line 370 to achieve adesired temperature at the water outlet 360. As noted above, thetemperature of the hot water exiting the first and second water tanks312, 330 may be hotter than the desired outlet 360 water temperature andthus the cold water may be used to temper, or cool, the hot waterleaving the tanks. While the system as shown as having a cold waterbranch line 346, a separate cold water line may be connected to themixing valve 356.

It is contemplated that the water temperature exiting the water outlet360 and the water temperature in the first and second water tanks 312,330 may be selected to be any value desired based on the particularapplication. The operating setpoint of the water temperature within thewater tanks 312, 330 may be selected based on a number of variables,such as, but not limited to, the temperature of the cold water enteringthe first and second water tank 312, 330, the desired water temperatureat the water outlet 360, the volume of the water tanks 312, 330 and/orthe percent of usable hot water in the water tanks 312, 330, etc. It isfurther contemplated that the proportion of cold water entering mixingvalve 356 from the branch line 346 to the hot water entering the mixingvalve 356 from the hot water line 370 may also be dependent upon anumber of variables, such as, but not limited to, the temperature of thecold water entering the mixing valve 356, the temperature of the hotwater entering the mixing valve 356, the desired water temperature atthe water outlet 360, etc. In some cases, the operating setpoints and/orthe proportion of cold water entering mixing valve 356 may be dynamic,and may vary with time, the temperature of the cold water entering thefirst water tank 312, the desired water temperature at the mixed wateroutlet 360, the volume of the water tanks 312, 330, the percent ofusable hot water in the water tanks 312, 330, and/or any other variable,as desired.

The mixed water outlet 360 may include an outlet water temperaturesensor 358 for measuring the temperature of the water exiting the waterheater system 300. In some embodiments, a temperature sensor 352 may beprovided in the hot water line 332 and a temperature sensor 354 may beprovided in the cold water line 348 to help improve control of the watertemperature exiting the water heater system 300 at the outlet line 360.While not explicitly shown, it is contemplated that a temperature sensormay also be provided in hot water line 316 and/or hot water line 370. Itis contemplated that the mixing valve 356 and the outlet watertemperature sensor 358 may be connected to the control units 318, 334via a communication bus 364, or other alternative connection. Forexample, it is contemplated that the mixing valve 356 and temperaturesensor 358 may be in wired or wireless communication with the controlunits 318, 334. Similarly, temperature sensors 328, 344, 352, 354 may bein communication with the control units 318, 334 via a communication bus364 or other alternative connection. It is contemplated that thetemperature sensors 328, 344, 352, 354 may be in wired or wirelesscommunication with the control units 318, 334. The water heater system300 may include an optional Man-Machine Interface (MMI) or userinterface 362. The user interface 362 may allow a user to view and/ormodify temperature setpoints and other control features.

In some instances, the gas burner 324 on the first water tank 312 may becontrolled by the control unit 318, and the gas burner 340 on the secondwater tank 330 may be controlled by the control unit 334. The firstcontrol unit 318 and the second control unit 334 may be in either wiredor wireless communication with one another to facilitate optimum controlof the entire water heater system 300. In such an arrangement, forexample, one control may act as master and the other as slave. Thiscommunication may allow for sharing of water temperature, safety status,water temperature stacking, etc. If a master/slave protocol is used, themaster may be allowed to increase or decrease the temperaturesetpoint(s) of itself and the slave controls. In some instances, themaster control may be allowed to lockout any of the controls, dependingon the safety status that was communicated to the master. It is alsocontemplated that a single overall control module (not explicitly shown)might be used for both tanks 312, 330, especially if both tanks 312, 330are part of the same assembly (not explicitly shown).

Referring briefly to FIG. 8, it will be appreciated that control units318, 334 may include controllers. FIG. 8 is a block diagram of such acontroller 700. The controller 700 may be considered as being a portionof control units 318, 334, or separate from control units 318, 334.Controller 700 may have several blocks. In some cases, controller 700may have an INPUT/OUTPUT block 710 that accepts signals from thetemperature sensors 328, 344, 352, 354, 358 (FIG. 4). If water heatersystem 300 is in communication with an external thermostat or othercontroller, INPUT/OUTPUT block 710 may accommodate externally-derivedcontrol signals, and/or provide status and/or other information, asdesired. In some cases, INPUT/OUTPUT block 710 may also provideappropriate output command signals to an electrically controlled gasvalve (not illustrated) within control units 318, 334, or otherelectrically controlled valve, such as mixing valve 356.

In some instances, controller may include a microprocessor 720 that maybe configured to accept appropriate signals from INPUT/OUTPUT block 710,and to determine appropriate output signals that can be outputted viaINPUT/OUTPUT block 710, such as to other components within control units318, 334 (FIG. 4) and/or to an external thermostat or other controller.Microprocessor 720 may be programmed to accept a temperature signal fromtemperature sensors 328, 344, 352, 354, 358 (FIG. 4), and to calculateor otherwise determine a command temperature that alters the temperaturevalue received from the temperature sensors 328, 344, 352, 354, 358 inorder to account or compensate for temperature differentials and/orthermal lag caused by the partial thermal isolation (if present) of thetemperature sensors 328, 344, 352, 354, 358 from the water in the watertanks 312, 330 or water lines 348, 332, 360. The microprocessor 720 maybe also be programmed to determine a command temperature based on thecurrent or expected hot water demand on the water heater system 300.While not explicitly illustrated, microprocessor 720 may also includememory and/or other components.

It is contemplated that one or both of the control units 318, 334 mayinclude a control algorithm for operating the mixing valve 356 based onthe desired water temperature at the water outlet 360. For example, thecontrol units 318, 334 may include a controller configured to providesignals to the mixing valve 356 via communication line 364 to change theratio of hot water to cold water based on feedback received from theuser interface 362, the outlet water temperature sensor 358, the coldwater temperature sensor 354, and/or the hot water temperature sensor352. While the mixing valve 356 has been described as an electronicallycontrollable mixing valve, it is contemplated that the mixing valve 356may be a non-electronically controlled mixing valve. It is contemplatedthat one or more mixing valves 356 may be used with any size of tank andwith any number of tanks.

The hot water capacity may depend on the temperature setpoint of thewater within the water tanks 312, 330, the temperature of the cold waterentering the first water tank 312 through the cold water inlet 348, thetemperature of the water exiting the first water tank 312 through thewater line 316, the temperature of the water exiting the second watertank 330 through the water line 332, and/or the desired temperature ofthe water exiting the water outlet 360.

It is contemplated that during use of the water heater system 300, hotwater may be drawn from both the first and second water tanks 312, 330evenly. However, this is not required. While not explicitly shown inFIG. 4, in some instances one or both of the hot water lines 316, 332may include an electronically controlled shut off valve. If a predictedhot water demand for a given time period is less than the volume of oneof tanks 312, 330, water may be drawn from only one of the first orsecond tanks 312, 330 by closing the shut off valve on the hot wateroutlet 316, 332.

It is further contemplated that in some instances, the first water tank312 may include a flue gas condensing heat exchanger. In someembodiments the flue gas exiting the second water tank 330 may bedirected through the flue 342 and into a heat exchanger (not explicitlyshown) in the first water tank 312. It is contemplated that the heatfrom the flue gas in flue 342 may be used in addition to or in place ofthe gas burner 324. If the flue gas heat exchanger is used in place ofthe gas burner 324, it is contemplated that the gas burner 324,combustion gas line 322, gas source 320, and flue 326 in the first watertank 312 may not be necessary. The reverse configuration may also beused.

FIG. 5 illustrates another exemplary water heater system 400. The waterheater system 400 may include a first water tank 412 and a second watertank 430 plumbed in parallel with one another including multi-stagemixing valves. While the system 400 is illustrated as having two tanks412, 430, it is contemplated that the system 400 may include any numberof tanks desired, such as, but not limited to three, four, or more. Insome embodiments, the first and second water tanks 412, 430 may be twosmaller tanks (for example, the tanks 412, 430 may each be less than 60gallons, less than 50 gallons, less than 40 gallons, less than 30gallons, etc.). It is contemplated that the first water tank 412 and thesecond water tank 430 may have the same volume. However, this is notrequired. In some instances, the first water tank 412 may have a largertank volume than the second water tank 430 or the second water tank 430may have a larger tank volume than the first water tank 412. The firstwater tank 412 and second water tank 430 may be standard (e.g.non-condensing) atmospheric water tanks or any form of storage watertanks desired. In some instances, the first water tank 412 and thesecond water tank 430 may be physically separate structures as shown inFIG. 5. In other instances, the first water tank 412 and the secondwater tank 430 may be fluidly isolated volumes that are part of the sameassembly. It is contemplated that, in some embodiments, one or both ofthe tanks 412, 430 may be tankless or instantaneous water heaters.

The first water tank 412 may include an insulating layer (not explicitlyshown) positioned about the water tank 412 to help reduce thermal lossesfrom the water tank 412. Cold water enters water tank 412 through a coldwater line 414 and is heated by a gas burner 424. In some cases, thewater tank 412 may include an electric heating element rather than a gasburner 424. When so provided, a power delivery unit (not shown) may beused to selectively apply power (i.e. current) to the electric heatingelement. For gas-fired water heaters, a control unit 418 may control agas control unit, such as a gas valve, to regulate gas flow from a gassource 420 through a combustion gas line 422 and into gas burner 424. Insome instances, control unit 418 may also control additional componentsof the water heater system. A flue 426 permits combustion byproducts tosafely exit. Whether oil, gas or electric powered, the resulting heatedwater exits through a hot water line 416.

Similarly, the second water tank 430 may include an insulating layer(not explicitly shown) positioned about the water tank 430 to helpreduce thermal losses from the water tank 412. Cold water may enter thesecond water tank 430 through a cold water line 450 branched off of acold water line 414 and is heated by a gas burner 440. In someinstances, cold water may be supplied to the second water tank 430through a separate cold water supply. In some cases, the water tank 430may include an electric heating element rather than a gas burner 440.When so provided, a power delivery unit (not shown) may be used toselectively apply power (i.e. current) to the electric heating element.For gas-fired water heaters, a control unit 434 may control a gascontrol unit, such as a gas valve, to regulate gas flow from a gassource 436 through a combustion gas line 438 and into gas burner 444. Insome instances, control unit 434 may also control additional componentsof the water heater system. A flue 442 permits combustion byproducts tosafely exit. Whether oil, gas or electric powered, the resulting heatedwater exits through a hot water line 432. The first water tank 412 mayinclude a temperature sensor 428, and the second water tank 430 mayinclude a temperature sensor 444.

Both the first water tank 412 and the second water tank 430 may behooked up to a cold water line 414 for receiving cold water from a watersource. In some instances, the cold water line 414 may include a branchline 446 directly connected to an electronically controlled mixing valve458 for injecting some cold water into the mixing valve 458, as will bediscussed in more detail below, while some cold water passes throughanother portion of the line 448 and enters the first water tank 412through the cold water inlet 470. In some embodiments, the cold waterline 414 may also include a second branch cold water line 450 fordelivering cold water to the second water tank 430 as shown. In otherembodiments, a separate cold water supply line may be used to deliverwater to the second water tank 430. The first gas burner 424 may heatthe cold water entering the first water tank 412 to a firstpredetermined temperature setpoint, and the second gas burner 440 mayheat the cold water entering the second water tank 430 to a secondpredetermined temperature setpoint. In some embodiments, the first andsecond temperature setpoints may be equal or approximately equal;however, this is not required. It is contemplated that the temperaturesetpoint of the first and second water tanks 412, 430 may be equal to orgreater than the desired water temperature at the water outlet 472. Insome cases, the first and second temperature setpoints may be different.

Heated water may exit the first water tank 412 through hot water line416. Cold water may be injected into the hot water line 416 at anelectronically controlled mixing valve 458. As noted above, thetemperature of the hot water exiting the first water tank 412 may behotter than the desired outlet 472 water temperature and thus the coldwater may be used to temper, or cool, the hot water leaving the tanks.While the system is shown as having a branch cold water line 446, it iscontemplated that a separate cold water line may be connected to themixing valve 458. The tempered water may flow through an intermediatewater line 474 to a second electronically controlled mixing valve 462.The second mixing valve 462 may mix tempered water from the first mixingvalve 458 with hot water exiting the second water tank 430 through hotwater line 432. The electronically controlled mixing valve 462 mayinject a regulated amount of hot water from the second tank 430 into theintermediate water line 474 to achieve a desired temperature at thewater outlet 472. As noted above, the temperature of the hot waterexiting the second water tank 430 may be hotter than the desired outlet472 water temperature and thus the tempered water from intermediate line474 may be used to temper, or cool, the hot water leaving the secondtank 430. Alternatively, the hot water line 432 can be seen as warmingthe water in the intermediate line 474.

It is contemplated that the water temperature exiting the water outlet472 and the water temperature in the first and second water tanks 412,430 may be selected to be any value desired based on the particularapplication. The operating setpoint of the water temperature within thewater tanks 412, 430 may be selected based on a number of variables,such as, but not limited to, the temperature of the cold water enteringthe first and second water tanks 412, 430, the desired water temperatureat the water outlet 472, the volume of the water tanks 412, 430 and/orthe percent of usable hot water in the water tanks 412, 430, etc. It isfurther contemplated that the proportion of cold water entering mixingvalve 458 from the branch line 446 to the hot water entering the mixingvalve 458 from the hot water line 416 may also be dependent upon anumber of variables, such as, but not limited to, the temperature of thecold water entering the mixing valve 458, the temperature of the hotwater entering the mixing valve 458, the desired water temperature atthe water outlet 472, etc. The proportion of hot water entering thesecond mixing valve 462 from the hot water line 432 to the temperedwater entering the second mixing valve 462 from intermediate line 474may also be dependent upon a number of variables, such as, but notlimited to, the temperature of the water in intermediate line 474, thetemperature of the water in the hot water line 432, the desired watertemperature at the water outlet 472, etc.

The mixed water outlet 472 may include an outlet water temperaturesensor 464 for measuring the temperature of the water exiting the waterheater system 400. The intermediate water line 474 may also include atemperature sensor 460 for measuring the temperature of the waterexiting the first mixing valve 458. In some embodiments, a temperaturesensor 452 may be provided in the hot water line 432, a temperaturesensor 454 may be provided in the cold water line 448, and a temperaturesensor 456 may be provided in hot water line 416 to improve control ofthe water temperature exiting the water heater system 400 at the outletline 472, although this is not required. It is contemplated that themixing valves 458, 462, the outlet water temperature sensor 464, and theintermediate temperature sensor 460 may be connected to the controlunits 418, 434 via a communication bus 468, or other alternativeconnection. For example, it is contemplated that the mixing valves 458,462 and temperature sensors 460, 464 may be in wired or wirelesscommunication with the control units 418, 434. Similarly, temperaturesensors 428, 444, 452, 454, 456 may also be in communication with thecontrol units 418, 434 via a communication bus 468 or other alternativeconnection. It is contemplated that the temperature sensors 428, 444,452, 454, 456 may be in wired or wireless communication with the controlunits 418, 434. The water heater system 400 may include an optionalMan-machine interface (MMI) or user interface 466. The user interface466 may allow a user to view and/or modify temperature setpoints andother control features.

In some instances, the gas burner 424 on the first water tank 412 may becontrolled by control unit 418 and the gas burner 440 on the secondwater tank 430 may be controlled by control unit 434. The first controlunit 418 and the second control unit 434 may be in either wired orwireless communication with one another to facilitate optimum control ofthe entire water heater system 400. In such an arrangement, for example,one control may act as master and the other as slave. This communicationmay allow for sharing of water temperature, safety status, watertemperature stacking, etc. If a master/slave protocol is established,the master may be allowed to increase or decrease the temperaturesetpoint(s) of itself and the slave controls. In some instances, themaster control may also be allowed to lockout any of the controls,depending on the safety status that was communicated to the master. Itis also contemplated that a single overall control module (notexplicitly shown) might be used for both tanks 412, 430, especially ifboth tanks 412, 430 are part of the same assembly (not explicitlyshown).

Referring briefly to FIG. 8, it will be appreciated that control units418, 434 may include controllers. FIG. 8 is a block diagram of such acontroller 700. The controller 700 may be considered as being a portionof control unit 418, 434, or separate from control unit 418, 434.Controller 700 may have several modules. In some cases, controller 700may have an INPUT/OUTPUT block 710 that accepts signals from thetemperature sensors 428, 444, 452, 454, 456, 464 (FIG. 5). If waterheater system 400 is in communication with an external thermostat orother controller, INPUT/OUTPUT block 710 may accommodateexternally-derived control signals, and/or provide status and/or otherinformation, as desired. In some cases, INPUT/OUTPUT block 710 may alsoprovide appropriate output command signals to an electrically controlledgas valve (not illustrated) within control units 418, 434, or otherelectrically controlled valve, such as mixing valves 458, 462.

In some instances, controller may include a microprocessor 720 that maybe configured to accept appropriate signals from INPUT/OUTPUT block 710,and to determine appropriate output signals that can be outputted viaINPUT/OUTPUT block 710, such as to other components within control units418, 434 (FIG. 5) and/or to an external thermostat or other controller.Microprocessor 720 may be programmed to accept a temperature signal fromtemperature sensors 428, 444, 452, 454, 456, 464 (FIG. 5), and tocalculate or otherwise determine a command temperature that alters thetemperature value received from the temperature sensors 428, 444, 452,454, 464 in order to account or compensate for temperature differentialsand/or thermal lag caused by the partial thermal isolation (if present)of the temperature sensors 428, 444, 452, 454, 464 from the water in thewater tanks 412, 430 or water lines 416, 432, 448, 472, 474. Themicroprocessor 720 may be also be programmed to determine a commandtemperature based on the current or expected hot water demand on thewater heater system 400. While not explicitly illustrated,microprocessor 720 may also include memory and/or other components.

It is contemplated that one or both of the control units 418, 434 mayinclude a control algorithm for operating the mixing valves 458, 462based on the desired water temperature at the water outlet 472. Forexample, the control units 418, 434 may include a controller configuredto provide signals to the mixing valve 458 via communication line 468 tochange the ratio of hot water to cold water based on feedback receivedfrom the user interface 466, the intermediate water temperature sensor460, the cold water temperature sensor 454, and/or the hot watertemperature sensor 456. While the mixing valve 458 has been described asan electronically controllable mixing valve, it is contemplated that themixing valve 458 may be a non-electronically controlled mixing valve.The control units 418, 434 may also include a controller configured toprovide signals to the second mixing valve 462 via communication line468 to change the ratio of hot water to tempered water based on feedbackreceived from the user interface 466, the outlet water temperaturesensor 464, the intermediate water temperature sensor 460, and/or thehot water temperature sensor 452. While the mixing valve 462 has beendescribed as an electronically controllable mixing valve, it iscontemplated that the mixing valve 462 may be a non-electronicallycontrolled mixing valve.

The hot water capacity may depend on the temperature setpoint of thewater within the water tanks 412, 430, the temperature of the cold waterentering the first water tank 412 through the cold water inlet 448, thetemperature of the water exiting the first water tank 412 through thewater line 416, the temperature of the water exiting the second watertank 430 through the water line 432 and the desired temperature of thewater exiting the water outlet 472.

It is contemplated that during use of the water heater system 400, hotwater may be drawn from both the first and second water tanks 412, 430evenly. However, this is not required. For example, during periods ofexpected low water draw, the first water tank 412 may be maintained at alower set point temperature than the second water tank 430, and may beused to temper the hot water from the second water tank 430 via themixing valve 462. In this and other scenarios, hot water may be drawnfrom the first and second water tanks 412, 430 unevenly.

While not explicitly shown, in some instances, one or both of the hotwater lines 416, 432 may include an electronically controlled shut offvalve. If the predicted hot water demand for a given time period is lessthan the volume of one of tanks 412, 430, water may be drawn from onlyone of the first or second tanks 412, 430 by closing the shut off valveon the hot water outlet 416, 432 of the other tank. The temperature setpoint of the tank that is shut off by the shut off valve may be reducedto help conserve energy. In some cases, the controller(s) may switchwhich of the tanks 412, 430 is shut off by the shut off valve, sometimesin an effort to balance the load across the tanks 412 and 430 over time.

While not explicitly shown, it is contemplated that in some instances,the first water tank 412 may include a flue gas condensing heatexchanger. In some embodiments the flue gas exiting the second watertank 430 may be directed through the flue 442 and into a heat exchanger(not explicitly shown) in the first water tank 412. It is contemplatedthat the heat from the flue gas in flue 442 may be used in addition toor in place of the gas burner 424. If the flue gas heat exchanger isused in place of the gas burner 424, it is contemplated that the gasburner 424, combustion gas line 422, gas source 420, and flue 426 in thefirst water tank 412 may not be necessary. The reverse configuration mayalso be used.

FIG. 6 illustrates another exemplary water heater system 500. The waterheater system 500 may include a first water tank 512 and a second watertank 530 plumbed in parallel with one another including multi-stagemixing valves. While the system 500 is illustrated as having two tanks512, 530, it is contemplated that the system 500 may include any numberof tanks desired, such as, but not limited to three, four, or more. Insome embodiments, the first and second water tanks 512, 530 may be twosmaller tanks (for example, the tanks 512, 530 may each be less than 60gallons, less than 50 gallons, less than 40 gallons, less than 30gallons, etc.). It is contemplated that the first water tank 512 and thesecond water tank 530 may have the same volume. However, this is notrequired. In some instances, the first water tank 512 may have a largertank volume than the second water tank 530, or the second water tank 530may have a larger tank volume than the first water tank 512. The firstwater tank 512 and second water tank 530 may be standard (e.g.non-condensing) atmospheric water tanks or any form of storage watertanks desired. In some instances, the first water tank 512 and thesecond water tank 530 may be physically separate structures as shown inFIG. 6. In other instances, the first water tank 512 and the secondwater tank 530 may be fluidly isolated volumes that are part of the sameassembly. It is contemplated that, in some embodiments, one or both ofthe tanks 512, 530 may be tankless or instantaneous water heaters.

The first water tank 512 may include an insulating layer (not explicitlyshown) positioned about the water tank 512 to help reduce thermal lossesfrom the water tank 512. Cold water enters water tank 512 through a coldwater line 514 and is heated by a gas burner 524. In some cases, thewater tank 512 may include an electric heating element rather than a gasburner 524. When so provided, a power delivery unit (not shown) may beused to selectively apply power (i.e. current) to the electric heatingelement. For gas-fired water heaters, a control unit 518 may control agas control unit, such as a gas valve, to regulate gas flow from a gassource 520 through a combustion gas line 522 and into gas burner 524. Insome instances, control unit 518 may also control additional componentsof the water heater system. A flue 526 permits combustion byproducts tosafely exit. Whether oil, gas or electric powered, the resulting heatedwater exits through a hot water line 516.

Similarly, the second water tank 530 may include an insulating layer(not explicitly shown) positioned about the water tank 530 to helpreduce thermal losses from the water tank 512. Cold water may enter thesecond water tank 530 through a cold water line 550 branched off of acold water line 514 and is heated by a gas burner 540. In someinstances, cold water may be supplied to the second water tank 530through a separate cold water supply. In some cases, the water tank 530may include an electric heating element rather than a gas burner 540.When so provided, a power delivery unit (not shown) may be used toselectively apply power (i.e. current) to the electric heating element.For gas-fired water heaters, a control unit 534 may control a gascontrol unit, such as a gas valve, to regulate gas flow from a gassource 536 through a combustion gas line 538 and into gas burner 544. Insome instances, control unit 534 may also control additional componentsof the water heater system. A flue 542 permits combustion byproducts tosafely exit. Whether oil, gas or electric powered, the resulting heatedwater exits through a hot water line 432.

The first water tank 512 may include a temperature sensor 528, and thesecond water tank 530 may include a temperature sensor 544. Both thefirst water tank 512 and the second water tank 530 may be hooked up to acold water line 514 for receiving cold water from a source. In someinstances, the cold water line 514 may include a branch line 546directly connected to an electronically controlled mixing valve 562 forinjecting some cold water into the mixing valve 562, as will bediscussed in more detail below, while some cold water passes throughanother portion of the line 548 and enters the first water tank 512through the cold water inlet 552. In some embodiments, the cold waterline 514 may also include a second cold water branch line 550 fordelivering cold water to the second water tank 530. In otherembodiments, a separate cold water supply line may be used to deliverwater to the second water tank 530 and/or a separate cold water supplyline may be used to deliver water to the mixing valve 562. The first gasburner 524 may heat the cold water entering the first water tank 512 toa first predetermined temperature setpoint, and the second gas burner540 may heat the cold water entering the second water tank 530 to asecond predetermined temperature setpoint. In some embodiments, thefirst and second temperature setpoints may be equal or approximatelyequal; however, this is not required. It is contemplated that thetemperature setpoint of the first and second water tanks 512, 530 may beequal to or greater than the desired water temperature at the wateroutlet 574. In some cases, the first and second temperature setpointsmay be different.

Heated water may exit the first water tank 512 through hot water line516. Cold water may be injected into the hot water line 516 at anelectronically controlled mixing valve 562. As noted above, thetemperature of the hot water exiting the first water tank 512 may behotter than the desired outlet 574 water temperature and thus the coldwater may be used to temper, or cool, the hot water leaving the tanks.While the system as shown as having a branch cold water line 446, it iscontemplated that a separate cold water line may be connected to themixing valve 562. The tempered water may flow through an intermediatewater line 580 to a secondary electronically controlled mixing valve566. The secondary mixing valve 566 may mix tempered water from thefirst mixing valve 562 with tempered water from a second electronicallycontrolled mixing valve 570.

Heated water may exit the second water tank 530 through hot water line532. Cold water may be injected into the hot water line 532 at anelectronically controlled mixing valve 570. In some embodiments, coldwater may be delivered to the mixing valve 570 through a cold water line554 branching off of cold water line 550. In other embodiments, coldwater may be delivered to the mixing valve 570 through a separate coldwater supply line. As noted above, the temperature of the hot waterexiting the second water tank 530 may be hotter than the desired outlet574 water temperature and thus the cold water may be used to temper, orcool, the hot water leaving the tanks. The tempered water may flowthrough an intermediate water line 582 to the secondary mixing valve566. The secondary mixing valve 566 may control the proportion of waterentering from intermediate line 580 to the water entering fromintermediate line 582 to achieve a desired temperature at the wateroutlet 574 based on the temperature of the water in each intermediateline 580, 582. In some instances, the water temperature in intermediateline 580 may be greater than, less than, or equal to the desired watertemperature at the outlet 574. The water temperature in intermediateline 582 may also be greater than, less than, or equal to the desiredwater temperature at the outlet 574. It is contemplated that if thetemperature of the water in either intermediate line 580, 582 is lessthan the desired water temperature at the outlet 574, the watertemperature in the other line 580, 582 may be greater than the desiredwater temperature at the outlet 574 in order to achieve the desiredwater temperature at the outlet 574.

It is contemplated that the water temperature exiting the water outlet574 and the water temperature in the first and second water tanks 512,530 may be selected to be any value desired based on the particularapplication. The operating setpoint of the water tanks 512, 530 may beselected based on a number of variables, such as, but not limited to,the temperature of the cold water entering the first and second watertank s 512, 530, the temperature of the cold water entering the secondwater tank 512 and the mixing valve 570, the desired water temperatureat the water outlet 574, the volume of the water tanks 512, 530 and/orthe percent of usable hot water in the water tanks 512, 530, etc. Theproportion of cold water entering mixing valves 562, 570 to the hotwater entering mixing valves 562, 570 from the tanks 512, 530 may alsobe dependent on a number of variables, such as, but not limited to, thetemperature of the cold water entering the mixing valves 562, 570, thetemperature of the hot water entering the mixing valves 562, 570, thedesired water temperature at the water outlet 574, etc. It is furthercontemplated that the proportion of water entering the secondary mixingvalve 566 from intermediate line 580 to the water entering the secondarymixing valve 566 from intermediate water line 582 may also be dependentupon a number of variables, such as, but not limited to, the temperatureof the water in intermediate lines 580, 582 and the desired watertemperature at the water outlet 574, etc.

The mixed water outlet 574 may include an outlet water temperaturesensor 572 for measuring the temperature of the water exiting the waterheater system 500. Intermediate water line 580 may also include atemperature sensor 564 for measuring the temperature of the waterexiting the first mixing valve 562. Intermediate water line 582 may alsoinclude a temperature sensor 568 for measuring the temperature of thewater exiting the second mixing valve 570. In some embodiments, atemperature sensor 556 may be provided in hot water line 532, atemperature sensor 560 may be provided in cold water line 548, and atemperature sensor 558 may be provided in hot water line 516 to improvecontrol of the water temperature exiting the water heater system 500 atthe outlet line 574, although this is not required. It is contemplatedthat the mixing valves 562, 566, 570, the outlet water temperaturesensor 572, and the intermediate temperature sensors 564, 568 may beconnected to the control units 518, 534 via a communication bus 576, orother alternative connection. For example, it is contemplated that themixing valves 562, 566, 570 and temperature sensors 564, 568, 572 may bein wired or wireless communication with the control units 518, 534.Similarly, temperature sensors 528, 544, 556, 560, 558 may also be incommunication with the control units 518, 534 via a communication bus576 or other alternative connection. It is contemplated that thetemperature sensors 528, 544, 556, 558, 560 may be in wired or wirelesscommunication with the control units 518, 534. The water heater system500 may include an optional Man-Machine Interface (MMI) or userinterface 578. The user interface 578 may allow a user to view and/ormodify temperature setpoints and other control features.

In some instances, the gas burner 524 on the first water tank 512 may becontrolled by the first control unit 518 and the gas burner 540 on thesecond water tank 530 may be controlled by the second control unit 534.Control unit 518 and control unit 534 may be in either wired or wirelesscommunication with one another to facilitate optimum control of theentire water heater system 500. In such an arrangement, for example, onecontrol may act as master and the other as slave. This communication mayallow for sharing of water temperature, safety status, water temperaturestacking, etc. If a master/slave protocol is established, the master maybe allowed to increase or decrease the temperature setpoint(s) of itselfand the slave controls. In some instances, the master control may alsobe allowed to lockout any of the controls, depending on the safetystatus that was communicated to the master. It is also contemplated thata single overall control module (not explicitly shown) might be used forboth tanks 512, 530, especially if both tanks 512, 530 are part of thesame assembly (not explicitly shown).

Referring briefly to FIG. 8, it will be appreciated that control units518, 534 may include controllers. FIG. 8 is a block diagram of such acontroller 700. The controller 700 may be considered as being a portionof control units 518, 534, or separate from control units 518, 534.Controller 700 may have several modules. In some cases, controller 700may have an INPUT/OUTPUT block 710 that accepts signals from thetemperature sensors 528, 544, 556, 558, 560, 572 (FIG. 6). If waterheater system 500 is in communication with an external thermostat orother controller, INPUT/OUTPUT block 710 may accommodateexternally-derived control signals, and/or provide status and/or otherinformation, as desired. In some cases, INPUT/OUTPUT block 710 may alsoprovide appropriate output command signals to an electrically controlledgas valve (not illustrated) within control units 518, 534, or otherelectrically controlled valve, such as mixing valves 562, 566, 570.

In some instances, controller may include a microprocessor 720 that maybe configured to accept appropriate signals from INPUT/OUTPUT block 710,and to determine appropriate output signals that can be outputted viaINPUT/OUTPUT block 710, such as to other components within control units518, 534 (FIG. 6) and/or to an external thermostat or other controller.Microprocessor 720 may be programmed to accept a temperature signal fromtemperature sensors 528, 544, 556, 558, 560, 572 (FIG. 6), and tocalculate or otherwise determine a command temperature that alters thetemperature value received from the temperature sensors 528, 544, 556,558, 560, 572 in order to account or compensate for temperaturedifferentials and/or thermal lag caused by the partial thermal isolation(if present) of the temperature sensors 528, 544, 556, 558, 560, 572from the water in the water tanks 512, 530 or water lines 516, 532, 548,574, 580, 582. The microprocessor 720 may be also be programmed todetermine a command temperature based on the current or expected hotwater demand on the water heater system 500. While not explicitlyillustrated, microprocessor 720 may also include memory and/or othercomponents.

It is contemplated that one or both of the control units 518, 534 mayinclude a control algorithm for operating the mixing valves 562, 570based on the desired water temperature at the water outlet 574. Forexample, the control units 518, 534 may include a controller configuredto provide signals to the mixing valves 562, 570 via communication line576 to change the ratio of hot water to cold water based on feedbackreceived from the user interface 578, the intermediate water temperaturesensors 564, 568, the cold water temperature sensor 560, and/or the hotwater temperature sensors 556, 558. While the mixing valves 562, 570have been described as electronically controllable mixing valves, it iscontemplated that the mixing valves 562, 570 may be a non-electronicallycontrolled mixing valve. The control units 518, 534 may also include acontroller configured to provide signals to the secondary mixing valve566 via communication line 576 to change the ratio of tempered waterfrom line 580 to tempered water from line 582 based on feedback receivedfrom the user interface 578, the outlet water temperature sensor 572,and/or the intermediate water temperature sensors 564, 568. While themixing valve 566 has been described as an electronically controllablemixing valve, it is contemplated that the mixing valve 566 may be anon-electronically controlled mixing valve.

The hot water capacity may depend on the temperature setpoint of thewater within the water tanks 512, 530, the temperature of the cold waterentering the first and second water tanks 512, 530, the temperature ofthe water exiting the first water tank 512 through the water line 516,the temperature of the water exiting the second water tank 530 throughthe water line 532 and the desired temperature of the water exiting thewater outlet 574.

It is contemplated that during use of the water heater system 500, hotwater may be drawn from both the first and second water tanks 512, 530evenly. However, this is not required. While not explicitly shown, insome instances one or both of the hot water lines 516, 532 may includean electronically controlled shut off valve. If the predicted hot waterdemand for a given time period is less than the volume of one of tanks512, 530, water may be drawn from only one of the first or second tanks512, 530 by closing a shut off valve on one of the hot water outlets.

While not explicitly shown, it is further contemplated that in someinstances, the first water tank 512 may include a flue gas condensingheat exchanger. In some embodiments, the flue gas exiting the secondwater tank 530 may be directed through the flue 542 and into a heatexchanger (not explicitly shown) in the first water tank 512. It iscontemplated that the heat from the flue gas in flue 542 may be used inaddition to or in place of the gas burner 524. If the flue gas heatexchanger is used in place of the gas burner 524, it is contemplatedthat the gas burner 524, combustion gas line 522, gas source 520, andflue 526 in the first water tank 512 may not be necessary. The reverseconfiguration may also be used.

Referring to FIGS. 4-6, in some instances, the water heater system 300,400, 500 may use the relationship between hot water usage and the timeof day to determine the most efficient way to heat the first and secondtanks. The relationship between hot water usage and the time of day maybe entered into the user interface 362, 466, 578, if so provided,directly by a user. In other embodiments, the controllers 318, 334, 418,434, 518, 534 may utilize an algorithm that monitors hot water usage todevelop a schedule of expected hot water demand. For systems without awater flow rate sensor, the controller(s) 318, 334, 418, 434, 518, 534may indirectly estimate hot water usage by monitoring the watertemperature and call-for-heat signals. For systems with a water flowrate sensor, the controller(s) 318, 334, 418, 434, 518, 534 may use thatsignal to directly measure the hot water usage.

For water heaters with an integrated controller, the controllers maycommunicate as master/slaves, as discussed above, or via anothercommunication protocol to adjust the temperature setpoint of each watertank 312, 330, 412, 430, 512, 530. For water heater systems without anintegrated controller, a single controller may be used to individuallycontrol the temperature of the other water tanks. Based on theconfiguration of the water heaters (series/parallel), the requiredhot-water capacity at that time of day, and whether there are any mixingor shutoff valves at the water heater outlets, the temperature setpointsof each of the water tanks 312, 330, 412, 430, 512, 530 may be optimizedsuch that the water tanks 312, 330, 412, 430, 512, 530 as a group couldmeet the immediate hot-water demand of the user.

In one example, a system including two tanks in parallel, such as tanks312, 330, tanks 412, 430, or tanks 512, 530, the first tank 312, 412,512 and the second tank 330, 430, 530 each receive cold water with afirst temperature T1 and heats the water to a second temperature T2. Itis contemplated that T2 may be greater than or equal to the desiredwater temperature at the outlet 360, 472, 574. For illustrative purposesonly, the first and second tanks 312, 412, 512, 330, 430, 530 may eachhave a volume of 30 gallons. However, this is merely exemplary and isnot intended the limit or define the volume of the tanks. The tanks 312,412, 512, 330, 430, 530 may have any volume desired. If the predictedhot water demand for the user at a given time is 10 gallons, the mastercontrol may reduce the temperature setpoint of the first tank 312, 412,512 to T1 such that the burner is not used to heat the water. A shutoffvalve on the hot water outlet 316, 416, 516 of the first tank 312, 412,512 may be shut such that water from the first tank 312, 412, 512 doesnot leave the tank. While separate shutoff valves are not shown in FIGS.4-6, mixing valves 458 and 562 may be used for this purpose, if desired.The temperature setpoint of the second tank 330, 430, 530 may be left ata temperature that would provide 10 gallons of water at the desiredwater temperature at the outlet 360, 472, 574. It is contemplated thatif the temperature setpoint of the second tank 330, 430, 530 is greaterthan the desired water temperature at the outlet 360, 472, 574, thenless than 10 gallons of heated water may be drawn from the second tank330, 430, 530. Such a control algorithm may prevent the burner 324, 424,524 on the first tank 312, 412, 512 from heating the water in the firsttank 312, 412, 512 unnecessarily. It is contemplated that the reverseconfiguration may also be used.

If the predicted hot water demand for the user at this time is 45gallons, the master control may change the temperature setpoint of thefirst tank 312, 412, 512 to T2 and leave the temperature setpoint of thesecond tank 330, 430, 530 at T2. This may allow the system 300, 400, 500to provide more than 30 gallons of water.

FIGS. 7A and 7B illustrate another exemplary water heater system 600.The water heater system 600 may include a first water tank 612 and asecond water tank 630. The first and second water tanks 612, 630 may beplumbed to switch between a parallel configuration, as shown 7A, and aseries configuration, as shown in FIG. 7B. While the system 600 isillustrated as having two tanks 612, 630, it is contemplated that thesystem 600 may include any number of tanks desired, such as, but notlimited to three, four, or more. In some embodiments, the first andsecond water tanks 612, 630 may be two smaller tanks (for example, thetanks 612, 630 may each be less than 60 gallons, less than 50 gallons,less than 40 gallons, less than 30 gallons, etc.). It is contemplatedthat the first water tank 612 and the second water tank 630 may have thesame volume. However, this is not required. In some instances, the firstwater tank 612 may have a larger tank volume than the second water tank630 or the second water tank 630 may have a larger tank volume than thefirst water tank 612. The first water tank 612 and second water tank 630may be standard (e.g. non-condensing) atmospheric water tanks or anyform of storage water tanks desired. In some instances, the first watertank 612 and the second water tank 630 may be physically separatestructures as shown in FIGS. 7A and 7B. In other instances, the firstwater tank 612 and the second water tank 630 may be fluidly isolatedvolumes that are part of the same assembly. It is contemplated that, insome embodiments, one or both of the tanks 612, 630 may be tankless orinstantaneous water heaters.

The first water tank 612 may include an insulating layer (not explicitlyshown) positioned about the water tank 612 to help reduce thermal lossesfrom the water tank 612. Cold water enters water tank 612 through a coldwater line 614 and is heated by a gas burner 624. In some cases, thewater tank 612 may include an electric heating element rather than a gasburner 624. When so provided, a power delivery unit (not shown) may beused to selectively apply power (i.e. current) to the electric heatingelement. In either case, the resulting heated water exits through awater outlet line 616. The water outlet 616 may include a shutoff valve678 for switching between a parallel and a series configuration, as willbe discussed in more detail below. For gas-fired water heaters, acontrol unit 618 may control a gas control unit, such as a gas valve, toregulate gas flow from a gas source 620 through a combustion gas line622 and into gas burner 624. In some instances, control unit 618 mayalso control additional components of the water heater system. A flue626 permits combustion byproducts to safely exit. Whether oil, gas orelectric powered, the resulting heated water exits through a hot waterline 616.

Similarly, the second water tank 630 may include an insulating layer(not explicitly shown) positioned about the water tank 630 to helpreduce thermal losses from the water tank 612. When the water heatersystem 600 is operated with the tanks in parallel, cold water may enterthe second water tank 630 through a cold water line 648 branched off ofa cold water line 614 and is heated by a gas burner 640. In someinstances, cold water may be supplied to the second water tank 630through a separate cold water supply. When the water heater system 600is operated in series, warm and/or hot water may enter the second watertank 630 from the first water tank 612 through a branch water line 652off of hot water outlet 616 and is heated by a gas burner 640. The coldwater line 648 and the warm/hot water line 652 may each be provided witha shutoff valve 650, 654 to allow the system to be operated in either aparallel configuration or a series configuration as desired, as will bediscussed in more detail below. In some cases, the water tank 630 mayinclude an electric heating element rather than a gas burner 640. Whenso provided, a power delivery unit (not shown) may be used toselectively apply power (i.e. current) to the electric heating element.The hot water line 632 may include two branch lines 658, 662 each havinga shutoff valve 660, 664 for switching between a parallel and a seriesconfiguration, as will be discussed in more detail below. For gas-firedwater heaters, a control unit 634 may control a gas control unit, suchas a gas valve, to regulate gas flow from a gas source 636 through acombustion gas line 638 and into gas burner 640. In some instances,control unit 634 may also control additional components of the waterheater system. A flue 642 permits combustion byproducts to safely exit.Whether oil, gas or electric powered, the resulting heated water exitsthrough a hot water line 632. The first water tank 612 may include atemperature sensor 628 and the second water tank 630 may include atemperature sensor 644.

It is contemplated that the water heater system 600 may be operated ineither a parallel configuration or a series configuration, depending onthe current conditions. The system 600 may switch between configurationsby actuating one or more shutoff valves, as will be described herein.Referring specifically to FIG. 7A, in the parallel configuration, boththe first water tank 612 and the second water tank 630 may be configuredto receive cold water from a source through cold water line 614. In someinstances, the cold water line 614 may include a branch line 646directly connected to an electronically controlled mixing valve 668 forinjecting some cold water into the mixing valve 668, as will bediscussed in more detail below, while some cold water passes throughanother portion of the line 680 and enters the first water tank 612through the cold water inlet 682. In some embodiments, the cold waterline 614 may also include a second branch cold water line 648 fordelivering cold water to the second water tank 630. In otherembodiments, a separate cold water supply line may be used to deliverwater to the second water tank 630. As noted above, branch cold waterline 648 may be provided with a shutoff valve 650. When the first andsecond tanks 612, 630 are operated in a parallel configuration, it maybe desirable to have cold water entering both the first and second tanks612, 630 simultaneously. In the parallel configuration, valve 650 may bein the open position (not shaded in FIG. 7A) to allow cold water to flowinto the second tank 630. Valve 654 in outlet line 652 may be closed(shaded in FIG. 7A), thus preventing the warm and/or hot water exitingthe first tank 612 from entering the second tank 630. The first gasburner 624 may heat the cold water entering the first water tank 612 toa first predetermined temperature setpoint and the second gas burner 640may heat the cold water entering the second water tank 630 to a secondpredetermined temperature setpoint. In some embodiments, the first andsecond temperature setpoints may be equal or approximately equal;however, this is not required. It is contemplated that the temperaturesetpoint of the first and second water tanks 612, 630 may be equal to orgreater than the desired water temperature at the water outlet 672. Insome cases, the first and second temperature setpoints may be different.

Still referring to FIG. 7A, heated water may exit the first water tank612 through hot water line 616. Hot water line 616 may include a shutoffvalve 678 that may be in the open position to allow hot water to enteran electronically controlled mixing valve 668. Heated water may alsoexit the second water tank 630 through hot water line 632. As notedabove, the hot water line 632 may include two branch lines 658, 662. Thefirst branch line 658 may include a shutoff valve 660 and may join hotwater line 616 at intersection 684. The second branch line 662 mayinclude a shutoff valve 664 and may direct water to the mixing valve668. In the parallel configuration, the valve 660 in the first branchline 658 may be open while the valve 664 in the second branch line 662may be closed. This may direct the hot water from the second tank 630 tomix with the hot water from the first tank 612 at an intersection 684.The hot water from lines 658 and 616 may mix and be delivered to anelectronically controlled mixing valve 668 via hot water line 686. Coldwater may be injected into the hot water line 686 at the electronicallycontrolled mixing valve 668. As noted above, the temperature of the hotwater exiting the first and second water tanks 612, 630 may be hotterthan the desired outlet 672 water temperature and thus the cold watermay be used to temper, or cool, the hot water leaving the tanks. Whilethe system as shown as having a branch cold water line 646, it iscontemplated that a separate cold water line may be connected to themixing valve 668. Water at the desired temperature may exit the system600 at the water outlet 672.

Referring now to FIG. 7B, in the series configuration, the first watertank 612 may be hooked up to a cold water line 614 for receiving coldwater from a source. In some instances, the cold water line 614 mayinclude a branch line 646 directly connected to an electronicallycontrolled mixing valve 668 for injecting some cold water into themixing valve 668, as will be discussed in more detail below, while somecold water passes through another portion of the line 680 and enters thefirst water tank 612 through the cold water inlet 682 into the firsttank 612. In some embodiments, the cold water line 614 may also includea second branch cold water line 648 for delivering cold water to thesecond water tank 630. As noted above, the branch cold water line 648may be provided with a shutoff valve 650. When the system is operated ina series configuration, valve 650 may be closed (shaded in FIG. 7B) toprevent cold water from entering the second water tank 630.

The gas burner 624 may heat the cold water entering the first water tank612 to a first predetermined temperature setpoint. It is contemplatedthat the temperature setpoint of first water tank 612 may be greaterthan the temperature of the cold water entering the first water tank 612but less than the desired output temperature of the water heater system600 at the mixed water outlet line 672. The heated water may exit thefirst water tank 612 through the water outlet line 616. As noted above,water outlet 616 may include a branch line 652 having a shutoff valve654 for directing warm/hot water to the second water tank 630. In theseries configuration, valve 654 may be open (not shaded in FIG. 7B)while valve 678 in the water outlet line 616 is closed (shaded in FIG.7B) to allow warm water to pass into the second water tank 630.

Still referring to FIG. 7B, the gas burner 640 may heat the waterentering the second water tank 630 to a second predetermined temperaturesetpoint. It is contemplated that the second predetermined temperaturesetpoint may be greater than the first predetermined temperaturesetpoint of the first water tank 612. In some instances, the secondpredetermined temperature setpoint may be the desired hot water setpointfor the water heater system 600. In other instances, the secondpredetermined temperature setpoint may be higher than the desired watertemperature exiting the water heater system 600 at the mixed wateroutlet line 672. It is contemplated that by maintaining the watertemperature of the first water tank 612 at a lower temperature, thewater heater system 600 standby losses and the overall water heatersystem 600 efficiency rating may be improved.

Heated water may exit the second water tank 630 through hot water line632. As noted above, the hot water line 632 may include two branch lines658, 662. The first branch line 658 may include a shutoff valve 660 andmay join hot water line at intersection 684. The second branch line 662may include a shutoff valve 664 and may direct water directly to themixing valve 668. In the series configuration, the valve 660 in thefirst branch line 658 may be closed while the valve 664 in the secondbranch line 662 may be open. This may direct the hot water from thesecond tank 630 to the electronically controlled mixing valve 668 viahot water line 662. Cold water may be injected into the hot water line662 at the electronically controlled mixing valve 668. As noted above,the temperature of the hot water exiting the second water tank 630 maybe hotter than the desired outlet 672 water temperature and thus thecold water may be used to temper, or cool, the hot water leaving thetanks. While the system as shown as having a branch cold water line 646,it is contemplated that a separate cold water line may be connected tothe mixing valve 668. Water at the desired temperature may exit thesystem 600 at the water outlet 672.

Regardless of the configuration of the tanks (e.g. parallel or series),it is contemplated that the water temperature exiting the water outlet672 and the water temperature in the first and second water tanks 612,630 may be selected to be any value desired based on the particularapplication. The operating setpoint of the water temperature within thewater tanks 612, 630 may be selected based on a number of variables,such as, but not limited to, the temperature of the cold water enteringthe first and/or second water tanks 612, 630 and the mixing valve 668,the desired water temperature at the water outlet 672, the volume of thewater tanks 612, 630 and/or the percent of usable hot water in the watertanks 612, 630, etc. The proportion of cold water entering the mixingvalve 668 to the hot water entering the mixing valves 668 from eitherline 686 or line 662 may also be dependent on a number of variables,such as, but not limited to, the temperature of the cold water enteringthe mixing valve 668, the temperature of the hot water entering themixing valve 668, the desired water temperature at the water outlet 672,etc.

The mixed water outlet 672 may include an outlet water temperaturesensor 670 for measuring the temperature of the water exiting the waterheater system 600. In some embodiments, a temperature sensor 656 may beprovided in the hot water line 632, a temperature sensor 690 may beprovided in the cold water line 680, and a temperature sensor 688 may beprovided in hot water line 616 to improve control of the watertemperature exiting the water heater system 600 at the outlet line 672,although this is not required. It is contemplated that the mixing valve668 and the outlet water temperature sensor 670 may be connected to thecontrol units 618, 634 via a communication bus 676, or other alternativeconnection. For example, it is contemplated that the mixing valve 668and temperature sensors 656, 690, 670, 688 may be in wired or wirelesscommunication with the control units 618, 634. Similarly, temperaturesensors 628, 644, 656, 690, 688 may also be in communication with thecontrol units 618, 634 via a communication bus 676 or other alternativeconnection. It is contemplated that the temperature sensors 628, 644,656, 690, 688 may be in wired or wireless communication with the controlunits 618, 634. The water heater system 600 may include an optionalMan-machine interface (MMI) or user interface 674. The user interface674 may allow a user to view and/or modify temperature setpoints andother control features.

In some instances, the gas burner 624 on the first water tank 612 may becontrolled by the first control unit 618 and the gas burner 640 on thesecond water tank 630 may be controlled by the second control unit 634.Control unit 618 and control unit 634 may be in either wired or wirelesscommunication with one another to facilitate optimum control of theentire water heater system 600. In such an arrangement, for example, onecontrol may act as master and the other as slave. This communication mayallow for sharing of water temperature, safety status, water temperaturestacking, etc. If a master/slave protocol is established, the master maybe allowed to increase or decrease the temperature setpoint(s) of itselfand the slave controls. In some instances, the master control may alsobe allowed to lockout any of the controls, depending on the safetystatus that was communicated to the master. It is also contemplated thata single overall control module (not explicitly shown) might be used forboth tanks 612, 630, especially if both tanks 612, 630 are part of thesame assembly (not explicitly shown).

Referring briefly to FIG. 8, it will be appreciated that control units618, 634 may include controllers. FIG. 8 is a block diagram of such acontroller 700. The controller 700 may be considered as being a portionof control unit 618, 634, or separate from control unit 618, 634.Controller 700 may have several blocks. In some cases, controller 700may have an INPUT/OUTPUT block 710 that accepts signals from thetemperature sensors 628, 644, 656, 690, 670, 688 (FIGS. 7A and 7B). Ifwater heater system 600 is in communication with an external thermostator other controller, INPUT/OUTPUT block 710 may accommodateexternally-derived control signals, and/or provide status and/or otherinformation, as desired. In some cases, INPUT/OUTPUT block 710 may alsoprovide appropriate output command signals to an electrically controlledgas valve (not illustrated) within control units 618, 634, or otherelectrically controlled valve, such as mixing valve 668.

In some instances, controller may include a microprocessor 720 that maybe configured to accept appropriate signals from INPUT/OUTPUT block 710,and to determine appropriate output signals that can be outputted viaINPUT/OUTPUT block 710, such as to other components within control units618, 634 (FIGS. 7A and 7B) and/or to an external thermostat or othercontroller. Microprocessor 720 may be programmed to accept a temperaturesignal from temperature sensors 628, 644, 656, 690, 670, 688 (FIGS. 7Aand 7B), and to calculate or otherwise determine a command temperaturethat alters the temperature value received from the temperature sensors628, 644, 656, 690, 670, 688 in order to account or compensate fortemperature differentials and/or thermal lag caused by the partialthermal isolation (if present) of the temperature sensors 628, 644, 656,690, 670, 688 from the water in the water tanks 612, 630 or water lines616, 632, 680, 672, 616. The microprocessor 720 may be also beprogrammed to determine a command temperature based on the current orexpected hot water demand on the water heater system 600. While notexplicitly illustrated, microprocessor 720 may also include memoryand/or other components.

It is contemplated that one or both of the control units 618, 634 mayinclude a control algorithm for operating the mixing valve 668 based onthe desired water temperature at the water outlet 672. For example, thecontrol units 618, 634 may include a controller configured to providesignals to the mixing valve 668 via communication line 676 to change theratio of hot water to cold water based on feedback received from theuser interface 674, the outlet water temperature sensor 670, the coldwater temperature sensor 690, and/or the hot water temperature sensors656, 688. It is further contemplated that the controller may providesignals to electronically controlled shutoff valves 650, 654, 660, 664,678 to open and close based on the desired operating configuration. Thecontroller may also use a control algorithm to determine the appropriatetemperature setpoints of the first and second tanks 612, 630 based onthe configuration of the tanks 612, 630 (e.g. parallel or series). Whilethe mixing valve 668 and shutoff valves 650, 654, 660, 664, 678 havebeen described as electronically controllable mixing valves, it iscontemplated that the valves 668, 650, 654, 660, 664, 678 may benon-electronically controlled valves.

It is contemplated that any number of mixing valves may be used with anysize of tank and with any number of tanks. For each tank the mixingvalves are used with, the effective hot water capacity may be increased.The amount of increase in the hot water capacity may depend on thetemperature setpoint of the water within the water tanks 612, 630, thetemperature of the cold water entering the first and second water tanks612, 630, the temperature of the water exiting the first water tank 612through the water line 616, the temperature of the water exiting thesecond water tank 630 through the water line 632 and the desiredtemperature of the water exiting the water outlet 672.

While not explicitly shown, it is further contemplated that in someinstances, the first water tank 612 may include a flue gas condensingheat exchanger. In some embodiments the flue gas exiting the secondwater tank 630 may be directed through the flue 642 and into a heatexchanger (not explicitly shown) in the first water tank 612. It iscontemplated that the heat from the flue gas in flue 642 may be used inaddition to or in place of the gas burner 624. If the flue gas heatexchanger is used in place of the gas burner 624, it is contemplatedthat the gas burner 624, combustion gas line 622, gas source 620, andflue 626 in the first water tank 612 may not be necessary. The reverseconfiguration may also be used.

As water heater systems may be provided in a parallel tankconfiguration, a series tank configuration, or capable of switchingbetween the two configurations, additional controls may be used tooptimized how the water heaters function in these different scenarios.FIG. 9 illustrates a method 800 for determining whether two or moretanks are in series or parallel. As noted above, the illustrative waterheater system may include controller having the necessarily hardware andsoftware to receive feedback from the water heaters as well as toprovide control signals to various components of the system. Thecontroller may also be configured to analyzed data received from thewater heater tanks to help optimize the performance of the system.

Using either wired or wireless communication between the individualwater heat controls, the control software may compare how the watertemperature on each tank behaves during water draw cycles. This may helpthe controller to determine if the tanks are plumbed in series or inparallel. For as system having two tanks (this is merely exemplary, itis contemplated that the system may have any number of tanks desired),the controller may monitor the changes in temperature (ΔT) in both thefirst and second tanks 802 during tank draws. The controller may thencompare the changes in temperature for the first tank and the secondtank to determine if they are approximately equal (or below a thresholdamount) 804. Simultaneous removal of water from both tanks may causecold water to enter the tanks at approximately the same rate causing thechanges in temperature to be approximately equal. Thus, the controllerdetermines the tanks are in parallel 806. The software may thendetermine a rule for the temperature setpoints of the first and secondtanks 808. It is contemplated that there may be several differentmethods of configuring the temperature setpoints of the parallel tanksusing the existing temperature setpoints. In one instance, thecontroller may apply the higher of two temperature setpoints to bothtanks. In other instances, the controller may apply the lower of twotemperature setpoints to both tanks. In yet another instance, thecontroller may average the two temperature setpoints and apply theaverage temperature setpoint to both tanks. These are merely examples.It is contemplated that once the control system has determined the tanksare in parallel any number of temperature setpoints may be applied tothe tanks. For example, it is not required that both tanks have sametemperature setpoint.

If the tanks are plumbed in series, the water temperature in the firsttank would change much more than the water temperature in the secondtank because cold water is replenishing the water drawn from the firsttank, while warmer water is replenishing the water drawn from the secondtank. Thus, if the tanks are plumbed in series, the changes intemperature for the first tank and the second tank are not approximatelyequal (e.g. greater than a threshold amount). Once the control softwaredetermines the changes in temperature for the first tank and the secondtank are not approximately equal (e.g. greater than the thresholdamount), the control software may verify the change in temperature ofthe first tank is greater than the temperature of the second tank 810.If the control software confirms the change in temperature of the firsttank is greater than the temperature of the second tank, it isdetermined that the tanks are in series 812. It is contemplated that ifchange in temperature of the first tank is not greater than the changein temperature of the second tank and the changes in temperatures arenot approximately equal, the water heater system may be incorrectlyinstalled. The control unit may provide an alert indicating furthersystem verification is necessary.

Once the control system determines the tanks are in series, the softwaremay then determine a rule for the temperature setpoints of the first andsecond tanks 814. In some instances, the software may apply a setpointto the second water tank that is greater to or equal to the desiredoutput temperature. The setpoint of the first tank may be a lowertemperature than the second tank to optimize the hot water capacity andbalance the work load between the two tanks. It is contemplated that thecontrol software may also take into consideration the presence or mixingvalves or multi-stage mixing valves, such as the ones described herein,when determining the temperature setpoints for the tanks in either theparallel or series configuration.

In some cases, the controller may control a setpoint temperature of thefirst tank and a setpoint temperature of the second tank in accordancewith a first control algorithm if the change in temperature in the firsttank is different from the change in temperature in the second tank byat least the threshold amount, and may control the setpoint temperatureof the first tank and the setpoint temperature of the second tank inaccordance with a second control algorithm that is different from thefirst control algorithm if the change in temperature in the first tankis not different from the change in temperature in the second tank by atleast the threshold amount.

The disclosure should not be considered limited to the particularexamples described above, but rather should be understood to cover allaspects of the disclosure as set out in the attached claims. Variousmodifications, equivalent processes, as well as numerous structures towhich the disclosure can be applicable will be readily apparent to thoseof skill in the art upon review of the instant disclosure.

What is claimed is:
 1. A water heater system for providing heated water to a hot water supply line of a building that distributes the heated water throughout the building, the water heater system comprising: a first water storage tank having a first water inlet and a first water outlet, the first water storage tank controlled to a first setpoint temperature; a second water storage tank having a second water inlet and a second water outlet, the second water storage tank controlled to a second setpoint temperature; a water line fluidly connecting the first water outlet to the second water inlet for transporting water from the first water storage tank to the second water storage tank; a mixing valve having a first inlet fluidly coupled to the second water outlet downstream of the second water storage tank, the mixing valve being controllable via a communications interface; a cold water supply line in fluid communication with a second inlet of the mixing valve; a mixed water outlet fluidly coupled to an outlet of the mixing valve, the mixed water outlet configured to be plumbed to the hot water supply line of the building; a controller for controlling the mixing valve via the communications interface; wherein the controller is further configured to monitor hot water usage from the second water storage tank to develop a hot water usage schedule of expected hot water demand; and wherein the controller is further configured to adjust the first setpoint temperature of the first water storage tank in accordance with the hot water usage schedule.
 2. The water heater system of claim 1, further comprising a temperature sensor positioned in the mixed water outlet and in communication with the controller.
 3. The water heater system of claim 2, wherein the controller is configured to adjust a ratio of cold water entering the mixing valve from the cold water supply line to heated water entering the mixing valve from the second water outlet to achieve a desired temperature at the mixed water outlet.
 4. The water heater system of claim 3, wherein the controller is configured to adjust the ratio of cold water entering the mixing valve from the cold water supply line to heated water entering the mixing valve from the second water outlet based on feedback received from the temperature sensor.
 5. The water heater system of claim 1, wherein the first setpoint temperature of the first water storage tank is lower than the second setpoint temperature of the second water storage tank, and the second setpoint temperature of the second water storage tank is greater than a desired outlet water temperature of the water heater system.
 6. The water heater system of claim 1, further comprising a temperature sensor positioned in the cold water supply line and in communication with the controller.
 7. A water heater system for providing heated water to a hot water supply line of a building that distributes the heated water throughout the building, the water heater system comprising: a first water storage tank having a first water inlet and a first water outlet, the first water storage tank controlled to a first setpoint temperature; a second water storage tank having a second water inlet and a second water outlet, the second water storage tank controlled to a second setpoint temperature; a water line fluidly connecting the first water outlet to the second water inlet for transporting water from the first water storage tank to the second water storage tank; a mixing valve having a first inlet fluidly coupled to the second water outlet downstream of the second water storage tank, the mixing valve being controllable via a communications interface; a mixed water outlet fluidly coupled to an outlet of the mixing valve; a controller for controlling the mixing valve via the communications interface; and wherein the controller is further configured to store a hot water usage schedule of expected hot water demand, and to adjust the first setpoint temperature of the first water storage tank in accordance with the hot water usage schedule.
 8. The water heater system of claim 7, wherein the controller is further configured to adjust the second setpoint temperature of the second water storage tank in accordance with the hot water usage schedule.
 9. The water heater system of claim 7, wherein the controller is configured to monitor a hot water usage of the water heater system to develop the hot water usage schedule.
 10. The water heater system of claim 7, wherein the controller is configured to control a gas control unit of the first water storage tank in accordance with the first setpoint temperature.
 11. The water heater system of claim 7, wherein the controller is configured to control a gas control unit of the second water storage tank in accordance with the second setpoint temperature.
 12. The water heater system of claim 7, wherein the controller is configured to control a gas control unit of the first water storage tank in accordance with the first setpoint temperature, and to control a gas control unit of the second water storage tank in accordance with the second setpoint temperature.
 13. The water heater system of claim 7, further comprising a temperature sensor positioned in the mixed water outlet and in communication with the controller, and wherein the controller adjusts a ratio of cold water entering the mixing valve from a cold water supply line to heated water entering the mixing valve from the second water outlet to achieve a desired temperature at the mixed water outlet.
 14. A water heater system for providing heated water to a hot water supply line of a building that distributes the heated water throughout the building, the water heater system comprising: a first water storage tank having a first water inlet and a first water outlet, the first water storage tank controlled to a first setpoint temperature; a second water storage tank having a second water inlet and a second water outlet, the second water storage tank controlled to a second setpoint temperature; a water line fluidly connecting the first water outlet to the second water inlet for transporting water from the first water storage tank to the second water storage tank; a mixing valve having a first inlet fluidly coupled to the second water outlet downstream of the second water storage tank; a mixed water outlet fluidly coupled to an outlet of the mixing valve; a controller configured to store a hot water usage schedule of expected hot water demand, and to adjust the first setpoint temperature of the first water storage tank in accordance with the hot water usage schedule.
 15. The water heater system of claim 14, wherein the controller is further configured to adjust the second setpoint temperature of the second water storage tank in accordance with the hot water usage schedule.
 16. The water heater system of claim 14, wherein the controller is configured to monitor a hot water usage of the water heater system to develop the hot water usage schedule.
 17. The water heater system of claim 14, wherein the controller is configured to control a gas control unit of the first water storage tank in accordance with the first setpoint temperature.
 18. The water heater system of claim 14, wherein the controller is configured to control a gas control unit of the second water storage tank in accordance with the second setpoint temperature.
 19. The water heater system of claim 14, wherein the controller is configured to control a gas control unit of the first water storage tank in accordance with the first setpoint temperature, and to control a gas control unit of the second water storage tank in accordance with the second setpoint temperature.
 20. The water heater system of claim 14, wherein the controller is further configured to control the mixing valve to achieve a desired water temperature at the mixed water outlet. 